make: support native clang for FreeBSD 10.0 and up

/etc/make.conf should contain:

    CC=clang
    CXX=clang++
    CPP=clang-cpp
    WITH_LIBCPLUSPLUS=yes
    WITH_CLANG=yes
    WITH_CLANG_IS_CC=yes
    WITH_CLANG_EXTRAS=yes
    WERROR=
    NO_WERROR=
    NO_FSCHG=
    WITHOUT_GCC=yes
    WITHOUT_GNU=yes

Compile with `gmake` from ports/packages