cmake: do not use $(MAKE) in Buildpmem.cmake

cmake: do not use $(MAKE) in Buildpmem.cmake

we cannot assume that we are using `make` as the cmake generatator,
for instance, if ninja is used, `$(MAKE)` won't be substituted by ninja.
so we need to check if Make is used as generator, if that's the case, we
can just use `$(MAKE)` so we can benefit from the job control of `make`,
otherwise, `make` is used, because currently, PMDK uses Makefile to
build.

Signed-off-by: Kefu Chai <kchai@redhat.com>