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
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