Use c99 instead of c11 for buildoptions test
Older verisons of MSVC do not support C11 properly.
-project('introspect buildargs', ['c'], default_options: ['c_std=c11', 'cpp_std=c++14', 'buildtype=release'])
+project('introspect buildargs', ['c'], default_options: ['c_std=c99', 'cpp_std=c++14', 'buildtype=release'])
subA = subproject('projectA')
