remove macos-gcc and linux-clang compiler configurations from tests

[peter-urban]

See #77

This is a first measure to reduce the number of test cases:

• I removed all macos - gcc configurations (calling gcc on macos seems to call clang anyways)
• I removed all linux - clang configurations (I think it is safe enough to assume that clang works similar enough on linux and mac. Clang compiler problems on linu,x that are not found on mac should probably be reported to llvm directly and not to GSW-C

[efiring]

I'm confused: it looks like you removed configurations from the Make suite, and added them to the meson suite.

[peter-urban]

The added lines in the meson test suit are in the exclude section :-)
I agree that having this exclude section is unnecessary complicated, but I'd prefer to simplify this in a later PR