Follow-up r288218 by ensuring common objects are built before recursing.

Follow-up r288218 by ensuring common objects are built before recursing.

Some example where this is a problem:
  lib/atf/libatf-c++/tests/Makefile:SRCS.${_T}=   ${_T}.cpp test_helpers.cpp
  lib/atf/libatf-c++/tests/detail/Makefile:SRCS.${_T}=    ${_T}.cpp test_helpers.cpp
  lib/atf/libatf-c/tests/Makefile:SRCS.${_T}=     ${_T}.c test_helpers.c
  lib/atf/libatf-c/tests/detail/Makefile:SRCS.${_T}=      ${_T}.c test_helpers.c
  lib/libpam/libpam/tests/Makefile:SRCS.${test} = ${test}.c ${COMMONSRC}

A similar change may be needed for FILES, SCRIPTS, or INCS, but for now stay
with just SRCS.

Reported by:	rodrigc
MFC after:	3 weeks
X-MFC-With:	r288218
Sponsored by:	EMC / Isilon Storage Division