Use string-desc module from gnulib.

Use string-desc module from gnulib.

* autogen.sh (GNULIB_MODULES_TOOLS_FOR_SRC): Add string-desc. Remove memmem,
memrchr.
(GNULIB_MODULES_LIBGETTEXTPO): Likewise.
* gettext-tools/src/str-desc.h: Remove file.
* gettext-tools/src/str-desc.c: Remove file.
* gettext-tools/src/FILES: Update.
* gettext-tools/src/msgl-ascii.h: Include string-desc.h instead of str-desc.h.
Use string_desc_t instead of string_desc_ty.
* gettext-tools/src/msgl-ascii.c: Use string_desc_t instead of string_desc_ty.
* gettext-tools/src/msgl-iconv.h: Include string-desc.h instead of str-desc.h.
Use string_desc_t instead of string_desc_ty.
* gettext-tools/src/msgl-iconv.c: Include string-desc.h instead of str-desc.h.
Use string_desc_t instead of string_desc_ty.
* gettext-tools/src/xg-encoding.h: Include string-desc.h instead of str-desc.h.
Use string_desc_t instead of string_desc_ty.
* gettext-tools/src/xg-encoding.c (string_desc_from_current_source_encoding):
Use string_desc_t instead of string_desc_ty.
* gettext-tools/src/x-perl.c: Include string-desc.h instead of str-desc.h.
(extract_quotelike_pass1, extract_quotelike_pass1_utf8, extract_quotelike,
extract_triple_quotelike, interpolate_keywords): Use string_desc_t instead of
string_desc_ty.
* gettext-tools/src/Makefile.am (noinst_HEADERS): Remove str-desc.h.
(libgettextsrc_la_SOURCES): Remove str-desc.c.
* gettext-tools/libgettextpo/Makefile.am (libgettextpo_la_AUXSOURCES): Remove
str-desc.c.