ICU4C: Fix GCC warnings about unknown pragma when building with MSYS2 (Windows).
There are numerous warnings about unknown pragmas when building ICU4C with GCC under MSYS2.
In file included from number_patternstring.h:16,
number_decimfmtprops.h:25: warning: ignoring #pragma warning [-Wunknown-pragmas]
#pragma warning(suppress: 4661)
We should make the MSVC-specific pragmas conditional on _MSC_VER being defined so reduce the number of noisy warnings.