Fix support for building a PROG_CXX, and PROG, directly.

For example in lib/atf/libatf-c++/tests/detail it is now possible to
run 'make application_test'. This was intended to worked for PROGS,
but lacked support for PROGS_CXX.

Also fix redefining the main PROG target to recurse. This isn't needed
since the main process is setting PROG/PROG_CXX to handle it directly
via bsd.prog.mk.

MFC after: 3 weeks
Sponsored by: EMC / Isilon Storage Division