* c_function_attributes: remove 'protected' from 'visibility'
This doesn't exist on macos with the apple compiler, which always causes
failures.
Fixes#5530
* compilers: Add split visibility checks to has_function_attribute
These check for a single visibility at a time, rather than all four at
once. This allows for finer grained searches, and should make using
these checks safer across operating systems.
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.
Dylan Baker