compilers: add fetching of define list for clang

Simmilar to gcc, the list of pre-processor defines can be fetched with
`-dM -E` option. The way cpu_family is determined on linux relies on
this list.

Fixes incorrect value of cpu_family on linux, when crosscompiling:

```
CC="clang -m32" meson ./build
```

Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Co-authored-by: Dylan Baker <dylan@pnwbakers.com>

7 files changed, 42 insertions, 9 deletions

diff --git a/mesonbuild/compilers/c.py b/mesonbuild/compilers/c.py
index 1bc9e84..aac99b4 100644
--- a/mesonbuild/compilers/c.py
+++ b/mesonbuild/compilers/c.py
@@ -86,9 +86,10 @@ class ClangCCompiler(ClangCompiler, CCompiler):
     _C18_VERSION = '>=8.0.0'
 
     def __init__(self, exelist, version, for_machine: MachineChoice,
-                 is_cross, info: 'MachineInfo', exe_wrapper=None, **kwargs):
+                 is_cross, info: 'MachineInfo', exe_wrapper=None,
+                 defines: T.Optional[T.List[str]] = None, **kwargs):
         CCompiler.__init__(self, exelist, version, for_machine, is_cross, info, exe_wrapper, **kwargs)
-        ClangCompiler.__init__(self)
+        ClangCompiler.__init__(self, defines)
         default_warn_args = ['-Wall', '-Winvalid-pch']
         self.warn_args = {'0': [],
                           '1': default_warn_args,
diff --git a/mesonbuild/compilers/cpp.py b/mesonbuild/compilers/cpp.py
index f4bcfa9..478a68c 100644
--- a/mesonbuild/compilers/cpp.py
+++ b/mesonbuild/compilers/cpp.py
@@ -155,10 +155,11 @@ class CPPCompiler(CLikeCompiler, Compiler):
 
 class ClangCPPCompiler(ClangCompiler, CPPCompiler):
     def __init__(self, exelist, version, for_machine: MachineChoice,
-                 is_cross, info: 'MachineInfo', exe_wrapper=None, **kwargs):
+                 is_cross, info: 'MachineInfo', exe_wrapper=None,
+                 defines : T.Optional[T.List[str]] = None, **kwargs):
         CPPCompiler.__init__(self, exelist, version, for_machine, is_cross,
                              info, exe_wrapper, **kwargs)
-        ClangCompiler.__init__(self)
+        ClangCompiler.__init__(self, defines)
         default_warn_args = ['-Wall', '-Winvalid-pch', '-Wnon-virtual-dtor']
         self.warn_args = {'0': [],
                           '1': default_warn_args,
diff --git a/mesonbuild/compilers/fortran.py b/mesonbuild/compilers/fortran.py
index c155b5b..af83c0e56 100644
--- a/mesonbuild/compilers/fortran.py
+++ b/mesonbuild/compilers/fortran.py
@@ -424,7 +424,7 @@ class FlangFortranCompiler(ClangCompiler, FortranCompiler):
                  **kwargs):
         FortranCompiler.__init__(self, exelist, version, for_machine,
                                  is_cross, info, exe_wrapper, **kwargs)
-        ClangCompiler.__init__(self)
+        ClangCompiler.__init__(self, [])
         self.id = 'flang'
         default_warn_args = ['-Minform=inform']
         self.warn_args = {'0': [],
diff --git a/mesonbuild/compilers/mixins/clang.py b/mesonbuild/compilers/mixins/clang.py
index 1c0ee45..0ee10ad 100644
--- a/mesonbuild/compilers/mixins/clang.py
+++ b/mesonbuild/compilers/mixins/clang.py
@@ -42,9 +42,10 @@ clang_optimization_args = {
 }  # type: T.Dict[str, T.List[str]]
 
 class ClangCompiler(GnuLikeCompiler):
-    def __init__(self):
+    def __init__(self, defines: T.Optional[T.Dict[str, str]]):
         super().__init__()
         self.id = 'clang'
+        self.defines = defines or {}
         self.base_options.append('b_colorout')
         # TODO: this really should be part of the linker base_options, but
         # linkers don't have base_options.
@@ -56,6 +57,12 @@ class ClangCompiler(GnuLikeCompiler):
     def get_colorout_args(self, colortype: str) -> T.List[str]:
         return clang_color_args[colortype][:]
 
+    def has_builtin_define(self, define: str) -> bool:
+        return define in self.defines
+
+    def get_builtin_define(self, define: str) -> T.Optional[str]:
+        return self.defines.get(define)
+
     def get_optimization_args(self, optimization_level: str) -> T.List[str]:
         return clang_optimization_args[optimization_level]
 
diff --git a/mesonbuild/compilers/objc.py b/mesonbuild/compilers/objc.py
index 52d258d..d351c88 100644
--- a/mesonbuild/compilers/objc.py
+++ b/mesonbuild/compilers/objc.py
@@ -86,7 +86,7 @@ class ClangObjCCompiler(ClangCompiler, ObjCCompiler):
                  **kwargs):
         ObjCCompiler.__init__(self, exelist, version, for_machine, is_cross,
                               info, exe_wrapper, **kwargs)
-        ClangCompiler.__init__(self)
+        ClangCompiler.__init__(self, [])
         default_warn_args = ['-Wall', '-Winvalid-pch']
         self.warn_args = {'0': [],
                           '1': default_warn_args,
diff --git a/mesonbuild/compilers/objcpp.py b/mesonbuild/compilers/objcpp.py
index c8b422b..10555b4 100644
--- a/mesonbuild/compilers/objcpp.py
+++ b/mesonbuild/compilers/objcpp.py
@@ -84,7 +84,7 @@ class ClangObjCPPCompiler(ClangCompiler, ObjCPPCompiler):
                  is_cross, info: 'MachineInfo', exe_wrapper=None,
                  **kwargs):
         ObjCPPCompiler.__init__(self, exelist, version, for_machine, is_cross, info, exe_wrapper, **kwargs)
-        ClangCompiler.__init__(self)
+        ClangCompiler.__init__(self, [])
         default_warn_args = ['-Wall', '-Winvalid-pch', '-Wnon-virtual-dtor']
         self.warn_args = {'0': [],
                           '1': default_warn_args,
diff --git a/mesonbuild/environment.py b/mesonbuild/environment.py
index 8fad628..cb6ae7d 100644
--- a/mesonbuild/environment.py
+++ b/mesonbuild/environment.py
@@ -726,6 +726,28 @@ class Environment:
         minor = defines.get('__LCC_MINOR__', '0')
         return dot.join((generation, major, minor))
 
+    @staticmethod
+    def get_clang_compiler_defines(compiler):
+        """
+        Get the list of Clang pre-processor defines
+        """
+        args = compiler + ['-E', '-dM', '-']
+        p, output, error = Popen_safe(args, write='', stdin=subprocess.PIPE)
+        if p.returncode != 0:
+            raise EnvironmentException('Unable to get clang pre-processor defines:\n' + output + error)
+        defines = {}
+        for line in output.split('\n'):
+            if not line:
+                continue
+            d, *rest = line.split(' ', 2)
+            if d != '#define':
+                continue
+            if len(rest) == 1:
+                defines[rest] = True
+            if len(rest) == 2:
+                defines[rest[0]] = rest[1]
+        return defines
+
     def _get_compilers(self, lang, for_machine):
         '''
         The list of compilers is detected in the exact same way for
@@ -1043,6 +1065,8 @@ class Environment:
             if 'clang' in out:
                 linker = None
 
+                defines = self.get_clang_compiler_defines(compiler)
+
                 # Even if the for_machine is darwin, we could be using vanilla
                 # clang.
                 if 'Apple' in out:
@@ -1063,7 +1087,7 @@ class Environment:
 
                 return cls(
                     ccache + compiler, version, for_machine, is_cross, info,
-                    exe_wrap, full_version=full_version, linker=linker)
+                    exe_wrap, defines, full_version=full_version, linker=linker)
 
             if 'Intel(R) C++ Intel(R)' in err:
                 version = search_version(err)