Reported by Vincent Torri on the icu-support mailing list.
When building ICU4C with MSYS2 + mingw-w64 on Windows, GCC gives the following warning message:
In the C++ standard library headers for GCC on MinGW/MSYS2, the class std::atomic is declared as _GLIBCXX_VISIBILITY(default), which means that it is already "exported" or "visible" (similar to dllexport) if _GLIBCXX_HAVE_ATTRIBUTE_VISIBILITY is true.
(And _GLIBCXX_HAVE_ATTRIBUTE_VISIBILITY is true for GCC versions greater than 4.0).
The issue that the compiler is complaining about is that the U_COMMON_API macro, which serves to export (or make visible) the class has *no effect*.
In other words it is merely complaining that the macro is redundant.
We should suppress this warning for GCC, as it leads to many noisy warnings when building under MinGW/MSYS2.