Support parallel make check with GNU make 4.2+

Support parallel make check with GNU make 4.2+

I noticed that make -jN check would run make check-single when using GNU
make 4.2.1.

In the end, it turned out that this is due to this change from the make
4.2 NEWS file:

* The amount of parallelism can be determined by querying MAKEFLAGS, even when
  the job server is enabled (previously MAKEFLAGS would always contain only
  "-j", with no number, when job server was enabled).

The fix is trivial: just accept an optional arg to -j in Makefile.in
(saw_dash_j).  Tested on i386-pc-solaris2.11 with just make and make
-j/-jN with both make 3.82 and 4.2.1.

	* Makefile.in (saw_dash_j): Allow for GNU make 4.2+ passing -jN in
	MAKEFLAGS.