Fix AX_OPENMP performance regression

When moving from AC_OPENMP and AX_OPENMP we stopped AC_SUBST of OPENMP_CFLAGS and these haven't been used since. Fix that, but do that by only using the OPENMP_CFLAGS as needed, that is as compiler hints for vectorizing some loops in the gravity interactions and only requiring the OpenMP runtime when linking against an OpenMP FFTW library.

Fixes #865 (closed)

src/Makefile.am

@@ -16,7 +16,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 # Add the non-standard paths to the included library headers
-AM_CFLAGS = $(HDF5_CPPFLAGS) $(GSL_INCS) $(FFTW_INCS) $(NUMA_INCS) $(GRACKLE_INCS)  $(SUNDIALS_INCS) $(OPENMP_CFLAGS) $(CHEALPIX_CFLAGS)
+AM_CFLAGS = $(HDF5_CPPFLAGS) $(GSL_INCS) $(FFTW_INCS) $(NUMA_INCS) $(GRACKLE_INCS)  $(SUNDIALS_INCS) $(CHEALPIX_CFLAGS)
 
 # Assign a "safe" version number
 AM_LDFLAGS = $(HDF5_LDFLAGS) $(FFTW_LIBS)