All compiler flags are non-standard because the C standard does not concern itself with them at all.
That said, there are effectively two standard compiler interfaces: MSVC and GCC and everyone else emulates one or both of these.
Similarily, symbol visibility is not something that the C standard cares about because it doesn't even care about libraries in the first place. Again, for all platforms that have symbol visibility (e.g. PE and ELF based ones, although the details differ) there is a defacto standard for the compiler flags and attributes to control the visilibity: the MSVC and GCC extensions.
That said, there are effectively two standard compiler interfaces: MSVC and GCC and everyone else emulates one or both of these.
Similarily, symbol visibility is not something that the C standard cares about because it doesn't even care about libraries in the first place. Again, for all platforms that have symbol visibility (e.g. PE and ELF based ones, although the details differ) there is a defacto standard for the compiler flags and attributes to control the visilibity: the MSVC and GCC extensions.