meson: partial fix for building pzstd on MSVC

meson: partial fix for building pzstd on MSVC

It uses non-portable compiler options unconditionally. Elsewhere, we
check the compiler ID and only add the right ones, globally. Do the same
here.

NDEBUG can actually be handled by a core option, so while we are moving
things around, do so.

Unfortunately, this doesn't fix things entirely. The remaining issue is
not Meson's issue though -- MSVC simply does not like this source code
and somehow chokes on innocent code with the inscrutable "syntax error"
and "illegal token".