Filter out platform-specific flags in TestCompilerDef

In https://github.com/maven-nar/cpptasks-parallel/issues/2
Simon Herter noticed that the test suite fails with MinGW compilers.

The reason is that GNU C/C++ on MinGW needs to pass a -mwindows flag
which the test suite did not expect.

The -m family of GNU C/C++ flags is machine-specific anyway, so let's
just filter them out.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>