build: Remove expat dependency

build: Remove expat dependency

* DEPENDENCIES: Suggest libxml2 instead of expat.
* gnulib-local/lib/markup.c: New file.
* gnulib-local/lib/markup.h: New file.
* gnulib-local/modules/markup: New file.
* autogen.sh (GNULIB_MODULES_LIBGETTEXTPO): Add markup module.
* gettext-tools/configure.ac: Remove checks for expat.
* gettext-tools/gnulib-lib/.gitignore: Ignore modules pulled by
gnulib-tool due to the markup module usage.
* gettext-tools/gnulib-tests/.gitignore: Likewise.
* gettext-tools/libgettextpo/.gitignore: Likewise.
* gettext-tools/libgettextpo/Makefile.am (libgettextpo_la_AUXSOURCES):
Remove ../src/libexpat-compat.c.
(libgettextpo_la_LDFLAGS): Remove @LTLIBEXPAT@.
* gettext-tools/src/Makefile.am (noinst_HEADERS): Remove
libexpat-compat.h.
(libgettextsrc_la_SOURCES): Remove libexpat-compat.c.
(libgettextsrc_la_LDFLAGS): Remove @LTLIBEXPAT@.
* gettext-tools/src/format-kde-kuit.c: Use functions from markup.h, when
the file is being compiled as part of libgettextpo.  Otherwise use
libxml2.
* gettext-tools/src/libexpat-compat.c: Remove.
* gettext-tools/src/libexpat-compat.h: Remove.