They are supposed to be set to gcc-$GCC and g++-$GCC respectively, if
the suffix is defined, so do it.
Also use "compiler", rather than "CC", in the matrix for consistency
with .travis.yml and to avoid confusion between this property, which may
be empty, and CC environment variable, which is supposed to be always
defined.
Finally, show the path and the version of the compiler being used: this
was also done under Travis CI and there doesn't seem to be any reason
not to do it here.
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