Commit 3bc28fcc6b ("Squelch MSVC warning exporting subclasses of
runtime_error", 2019-11-29) silenced a MSVC warning under. The MinGW
compiler also defines _WIN32, but does not support the "warning" pragma.
Introduce a helper macro to squelch the MSVC warning only when using the
Microsoft compiler.
Signed-off-by: Beat Bolli <dev@drbeat.li>
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