Take advantage of Gnulib configmake module.

Gnulib provides the "configmake" module for discovering configuration
directories at build time.  PSPP already depends on this module indirectly,
so we might as well use it instead of defining our own variables in
Makefile.am and src/automake.mk.

(We can't replace INSTALLDIR by BINDIR because it is used under the
former name by the "relocatable-prog" module.)