Starting with GCC 10, let us enable _GLIBCXX_DEBUG as well as
_GLIBCXX_ASSERTIONS which have an impact on the GNU libstdc++.
On GCC 8, we observed a compilation failure related to some
missing type conversion.

Even though clang on GNU/Linux would default to using libstdc++
and enabling the debugging seems to work with clang-18, we will
not enable this on clang, in case it would lead to compilation
errors.

For the clang libc++ before clang-15 there was _LIBCPP_DEBUG,
but according to
llvm/llvm-project@f3966eaf869b7bdd9113ab9d5b78469eb0f5f028 and
llvm/llvm-project@13ea1343231fa4ae12fe9fba4c789728465783d7 and
llvm/llvm-project@ff573a42cd1f1d05508f165dc3e645a0ec17edb5 it
looks like that for proper results, a specially built debug version
of libc++ would have to be used in order to enable equivalent checks.

This should help catch bugs like the one that
commit 455a15fd fixed.

Reviewed by: Sergei Golubchik