Remove gsettings.m4 and write directly into Makefile.in

Remove gsettings.m4 and write directly into Makefile.in

* configure.ac (HAVE_PGTK): Declare substitution
(HAVE_GSETTINGS): Declare substitution
* Makefile.in (HAVE_PGTK): New variable
(HAVE_GSETTINGS): New variable
(gsettingsschemadir): Don't use value from gsettings.m4
(distclean): Add clean-gsettings-schemas
(install-gsettings-schemas): Implement instead of gsettings.m4
(uninstall-gsettings-schemas): Implement instead of gsettings.m4
(clean-gsettings-schemas): Implement instead of gsettings.m4
($(gsettings_SCHEMAS:.xml=.valid)): Implement instead of gsettings.m4
* m4/gsettings.m4: Remove