Adjust USES to fix building on gcc-based archs (in particular, powerpc64).

On the clang architectures, these changes have no effect: the baser
compiler already includes the needed features.

While here, remove now-redundant CXXFLAGS statements and pet portlint
(no other content change).

Tested on both powerpc64 and amd64.

Approved by: portmgr (tier-2 blanket)