This port needs USES=compiler:c++11-lang to build on GCC-based
architectures.

Additionally, our CXXFLAGS conflict with GCC. Set them only on Clang-
based architectures. -stdlib=libc++ isn't needed anymore, it's used
by default.

While here, also set USES=gnome.

PR: 234190
Submitted by: Piotr Kubaj
Approved by: portmgr (tier-2 blanket)