xgettext: Add support for generic XML files

xgettext: Add support for generic XML files

* autogen.sh (GNULIB_MODULES_TOOLS_FOR_SRC): Add trim module.
* gettext-tools/src/locating-rule.h: New file.
* gettext-tools/src/locating-rule.c: New file.
* gettext-tools/src/its.h: New file.
* gettext-tools/src/its.c: New file.
* gettext-tools/src/xgettext.c: Include "locating-rule.h" and "its.h".
(its_locators): New variable.
(long_options): Add --itstool option.
(extract_from_xml_file): New function.
(main): Handle --itstool option.
(usage): Document --itstool option.
* gettext-tools/src/Makefile.am (noinst_HEADERS): Add locating-rule.h
and its.h.
(libgettextsrc_la_CPPFLAGS): Add $(INCXML).
(libgettextsrc_la_SOURCES): Add locating-rule.c and its.c.
* gettext-tools/Makefile.am (SUBDIRS): Add its directory.
* gettext-tools/configure.ac: Output its/Makefile.
* gettext-tools/doc/gettext.texi (Preparing ITS Rules): New section.
* gettext-tools/doc/xgettext.texi: Mention --itstool option.
* gettext-tools/tests/Makefile.am (TESTS): Add new tests.
* gettext-tools/tests/xgettext-its-1: New file
* gettext-tools/tests/init-env.in: Set GETTEXTDATADIR for ITS tests.