build: Unify the three copies of texi2html.
* build-aux/texi2html: Moved to here from libtextstyle/build-aux/texi2html.
* libtextstyle/autogen.sh: Copy texi2html.
* gettext-runtime/libasprintf/texi2html: Remove file.
* gettext-runtime/libasprintf/Makefile.am (TEXI2HTML): Use build-aux/texi2html.
(EXTRA_DIST): Remove texi2html.
* gettext-tools/doc/texi2html: Remove file.
* gettext-tools/doc/Makefile.am (TEXI2HTML): Use build-aux/texi2html.
(EXTRA_DIST): Remove texi2html.
* Makefile.am (EXTRA_DIST): Add build-aux/texi2html.
(distcheck-hook): Update.
