has_function_attribute() depends on -Wattributes being emitted when an attribute
is not supported by the compiler. In case of GCC on Window (at least) there is no
warning in case the attribute is used on a declaration. Only once there is also a
function definition does it emit a warning like:
a.c: In function ‘foo’:
a.c:8:1: warning: visibility attribute not supported in this configuration; ignored [-Wattributes]
8 | }
To fix this add a dummy function definition to all visibility compiler checks in meson.
The tests in "197 function attributes" only checked for positive return result on on-msvc
compilers, except one special case for dllexport/dllimport. Refactor the tests a bit so
one can specify also a negative expected result, and add tests for all visibility attribute
variants.
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