Reported by Vincent Torri on themailing 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
(And is true for GCC versions greater than 4.0).
The issue that the compiler is complaining about is that themacro, 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.