Fix building in a directory with SUBDIRs and SUBDIR_PARALLEL.

The SUBDIR_PARALLEL feature uses a .for dir in ${SUBDIR} loop. The old code
here for recursing was setting SUBDIR= as a make *argument*. The SUBDIR=
replacement was not actually handled until after the .for loop was unrolled.
This could be seen with a '.info ${SUBDIR} ${dir}' inside of the loop which
showed an empty ${SUBDIR} and a set ${dir}. Setting NO_SUBIDR= before calling
${MAKE} as an *environment* variable handles the case fine and is a more
proper mechanism for disabling subdir handling.

This could be seen with 'make -C tests/sys/kern -j15 SUBDIR_PARALLEL=yes'.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division