If the .gschema.xml file was generated as the result of an implicit make
rule then make would 'rm' it after creating the validity stamp. This
would cause 'make install' to fail.
If --uninstall is given then don't give an error if the schema directory
is empty. Instead, erase the gschemas.compiled file, if it exists.
This is the right thing to do in the 'make uninstall' rule, where the
schema directory could very well be left empty as a result.
Modify gsettings.m4 to use this option.
Rename the --schema-files option to --schema-file, since it only
accepts one file at a time. Change the GSETTINGS_CHECK_RULE to
use it that way, too. And also make it work better with !srcdir
builds.
Bugs #616731 and #616864
