Extract libexpat compatibility layer to libexpat-compat.{c,h} files.

Move expat loading code out from x-glade.c to a separate file, so it
can be used by GSettings extractor.
xgettext: allow file name extension with multiple dots

To support extensions like ".gschema.xml", try possible extensions
until matching language is found.
xgettext: support GSettings schema file
Update after gnulib changed.
NEWS: Add entry for 0.18.4
Fix copyright year in xgettext version string.
Add .dir-locals.el to force space indentation.
Fix AC_CHECK_DECLS usage.
x-gsettings: add guard when expat is not available at compile time
glade: handle multi-line extracted comments
tests: fix make distcheck
autopoint: make macro trace in autom4te robuster
glade: Ignore --extract-all option for Glade2/3.
Update copyright year in Windows resource.