compilers: add logging to obscure compiler defines scraper

If this command fails, for example when CXX is something not generic
enough to be a valid universal compiler command (clang -std=c++11
perhaps), we end up with two problems:

- it's impossible to figure out what Meson ran to get that error

- the error report isn't clear on what is stdout and what is stderr, or
  even that that is what the message is about.

  ```
  meson.build:1:0: ERROR: Unable to get clang pre-processor defines:
  error: invalid argument '-std=c++11' not allowed with 'C'
  ```

  What's C doing there and why is Meson talking about it? Answer: that's
  compiler stdout. Say so.

1 files changed, 8 insertions, 2 deletions

diff --git a/mesonbuild/compilers/detect.py b/mesonbuild/compilers/detect.py
index d7929ad..6bc083d 100644
--- a/mesonbuild/compilers/detect.py
+++ b/mesonbuild/compilers/detect.py
@@ -1230,9 +1230,12 @@ def _get_gnu_compiler_defines(compiler: T.List[str]) -> T.Dict[str, str]:
     # Arguments to output compiler pre-processor defines to stdout
     # gcc, g++, and gfortran all support these arguments
     args = compiler + ['-E', '-dM', '-']
+    mlog.debug(f'Running command: {join_args(args)}')
     p, output, error = Popen_safe(args, write='', stdin=subprocess.PIPE)
     if p.returncode != 0:
-        raise EnvironmentException('Unable to detect GNU compiler type:\n' + output + error)
+        raise EnvironmentException('Unable to detect GNU compiler type:\n'
+                                   f'Compiler stdout:\n{output}\n-----\n'
+                                   f'Compiler stderr:\n{error}\n-----\n')
     # Parse several lines of the type:
     # `#define ___SOME_DEF some_value`
     # and extract `___SOME_DEF`
@@ -1254,9 +1257,12 @@ def _get_clang_compiler_defines(compiler: T.List[str]) -> T.Dict[str, str]:
     Get the list of Clang pre-processor defines
     """
     args = compiler + ['-E', '-dM', '-']
+    mlog.debug(f'Running command: {join_args(args)}')
     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)
+        raise EnvironmentException('Unable to get clang pre-processor defines:\n'
+                                   f'Compiler stdout:\n{output}\n-----\n'
+                                   f'Compiler stderr:\n{error}\n-----\n')
     defines: T.Dict[str, str] = {}
     for line in output.split('\n'):
         if not line: