MFC r240966 (by brooks)

Add a COMPILER_FEATURES variable that is set in bsd.compiler.mk.

When the compiler is clang add c++11 to COMPILER_FEATURES.

When the compiler supports c++11, default to building libc++.  This will
not impact the compliation of programs, but will make it easier for
users with clang compiled systems to test libc++ by passing -stdlib=libc++
to their c++ compliations.