Add method to check for C/C++ function attributes

It's fairly common on Linux and *BSD platforms to check for these
attributes existence, so it makes sense to me to have this checking
build into meson itself. Autotools also has a builtin for handling
these, and by building them in we can short circuit cases that we know
that these don't exist (MSVC).

Additionally this adds support for two common MSVC __declspec
attributes, dllimport and dllexport. This implements the declspec
version (even though GCC has an __attribute__ version that both it and
clang support), since GCC and Clang support the MSVC version as well.
Thus it seems reasonable to assume that most projects will use the
__declspec version over teh __attribute__ version.

1 files changed, 29 insertions, 0 deletions

diff --git a/docs/markdown/snippets/function_attributes.md b/docs/markdown/snippets/function_attributes.md
new file mode 100644
index 0000000..5514494
--- /dev/null
+++ b/docs/markdown/snippets/function_attributes.md
@@ -0,0 +1,29 @@
+## Helper methods added for checking GNU style attributes: __attribute__(...)
+
+A set of new helpers have been added to the C and C++ compiler objects for
+checking GNU style function attributes. These are not just simpler to use, they
+may be optimized to return fast on compilers that don't support these
+attributes. Currently this is true for MSVC.
+
+```meson
+cc = meson.get_compiler('c')
+if cc.has_function_attribute('aligned')
+   add_project_arguments('-DHAVE_ALIGNED', language : 'c')
+endif
+```
+
+Would replace code like:
+
+```meson
+if cc.compiles('''into foo(void) __attribute__((aligned(32)))''')
+   add_project_arguments('-DHAVE_ALIGNED', language : 'c')
+endif
+```
+
+Additionally, a multi argument version has been added:
+
+```meson
+foreach s : cc.get_supported_function_attributes(['hidden', 'alias'])
+   add_project_arguments('-DHAVE_@0@'.format(s.to_upper()), language : 'c')
+endforeach
+```