This is to see if it will stop intermittent build failures on buildfarm
member okapi.  We know that gmake 3.82 has some problems with sometimes
not honoring dependencies in parallel builds, and it seems likely that
this is more of the same.  Since the vast bulk of the work in the preproc
directory is associated with creating preproc.c and then preproc.o,
parallelism buys us hardly anything here anyway.

Also, make both this .NOTPARALLEL and the one previously added in
interfaces/ecpg/Makefile be conditional on "ifeq ($(MAKE_VERSION),3.82)".
The known bug in gmake is fixed upstream and should not be present in
3.83 and up, and there's no reason to think it affects older releases.