Add the cmm_cpp_is_gcc predicate to the testsuite

A future C-- test called T24474-cmm-override-g0 relies on the
GCC-specific behaviour of -g3 implying -dD, which, in turn, leads to it
emitting #defines past the preprocessing stage.  Clang, at least, does
not do this, so the test would fail if ran on Clang.

As the behaviour here being tested is ``-optCmmP-g3'' undoing effects of
the workaround we apply as a fix for bug #24474, and the workaround was
for GCC-specific behaviour, the test needs to be marked as fragile on
other compilers.