While GCC 4.x supports the usage of -fvisibility=hidden and the related
pragmas, usage of it under MinGW is a noop and should not be used.
Instead, __declspec(dllexport/dllimport) should be used instead to
indicate exportable functions.
Since Ninja builds a static library, no functions are exported and
visibility, at least under MinGW, is not an issue.
MinGW g++ recognizes -static as option to avoid the need to specify
individual libraries to be statically linked into the executable.
It also solves the warning of -static-libstdc++ not being recorgnized by
GCC 4.5.2 (TDM build)