Add USE_CXXSTD to fixes build on arches using GCC.

Set USES=localbase:ldflags, instead of manually specifying CPPFLAGS
and LDFLAGS.

Tested on powerpc64 (just this port), amd64 (this port plus libreoffice).

PR: 231447
Submitted by: Piotr Kubaj
Approved by: portmgr (tier-2 blanket)