As suggested in D27598. This also supports MK_WERROR.clang=no and
MK_WERROR.gcc=no to support the existing NO_WERROR.<compiler> uses.
Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D27601
Currently the googletest internal tests build after the matching library.
However, each of these is serialized at the top level makefile.
Additionally some of the tests (e.g. the gmock-matches-test) take up to
90 seconds to build with clang -O2. Having to wait for this test to
complete before continuing to the next directory seriously slows down the
parllelism of a -j32 build.
Before this change running `make -C lib/googletest -j32 -s` in buildenv
took 202 seconds, now it's 153 due to improved parallelism.
Reviewed By: emaste (no objection)
Differential Revision: https://reviews.freebsd.org/D26748
