contrib: gme: use CMAKE_CXX_IMPLICIT_LINK_LIBRARIES to get the C++ runtime

And since we need to patch the pkg-config, add libm from there as well.

Other cmake-based contribs compiling C++ code are already using the same trick (not always filtering the mingw mess)

Alternative fix to !2476 (closed). I'll try to send these patches upstream (but it's bitbucket 😨 ).

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v2:

• Check libm is actually needed before adding it. And hopefully this time it works.