Fix port builds for architectures which remain on gcc as the default compiler.
This will primarily help powerpc64.

For archs where clang is the default compiler, these changes will have no
effect, as the base compiler already has these capabilities (primarily, but
not exclusively, c++-11).

Tested for no-harm on amd64.

While here, pet portlint.

Approved by: portmgr (tier-2 blanket)