* Makefile.am: scmconfig.h is now generated by building and

* Makefile.am: scmconfig.h is now generated by building and
running gen-scmconfig.h and capturing its output.  gen-scmconfig
uses config.h and the configure.in generated gen-scmconfig.h to
decide what to output.  See gen-scmconfig.c for details.
(noinst_PROGRAMS): add gen-scmconfig.
(gen_scmconfig_SOURCES): new variable.
(gen-scmconfig.$(OBJEXT)): new target - be careful to handle
cross-compiling right.
(scmconfig.h): build scmconfig.h from gen-scmconfig's output.
(BUILT_SOURCES): add scmconfig.h.