[AC21.4] Parallel builds.

RECOMMEND 21.4 This is the 21.4 version of the my parallel builds patch. There is no reason to include this in 21.4.16 if its inclusion would delay the release. Malcolm ChangeLog addition: 2004-09-08 Malcolm Purvis <malcolmp(a)xemacs.org&gt; * INSTALL: A POSIX compatible Make is required for builds. * Makefile.in.in: Enable support for parallel builds. src/ChangeLog addition: 2004-09-08 Malcolm Purvis <malcolmp(a)xemacs.org&gt; * Makefile.in.in: Enable support for parallel builds.