make: build/run tests, only with --enable-tests

make: build/run tests, only with --enable-tests

Control the building and testing (make check) of the tests using the
if/endif WITH_TESTS guard based on the configuration options passed.
If the --enable-tests (default on) is passed during ./configure step,
the SUBDIRS directories (ftests/gunit) are recused into build and
make check are run.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>