Draft: Decrease default vectorization threshold when compiling gravity interactions

When optimizing with AVX2 some of the loops (notably at line 1092) are no longer vectorized. This makes that happen and gives good speed ups.

Fixes #865 (closed)

Now draft as the real solution could be a regression in how we use openmp pragmas as compilation hints.

configure.ac

@@ -586,7 +586,7 @@ if test "$enable_opt" = "yes" ; then
             GRAVITY_CFLAGS="$GRAVITY_CFLAGS -qopt-zmm-usage=high"
 	    ;;
 	 *)
-	    AC_MSG_WARN([No additional flags needed for gravity on this platform])
+            GRAVITY_CFLAGS="$GRAVITY_CFLAGS -vec-threshold=50"
 	    ;;
       esac
    elif test "$ax_cv_c_compiler_vendor" = "gnu"; then