gnulib-local: Replace libxml2 trio with Gnulib
* gettext-tools/gnulib-lib/.gitignore: Ignore files brought by
gnulib-tool.
* gettext-tools/gnulib-tests/.gitignore: Likewise.
* gnulib-local/lib/libxml/triodef.h: Remove.
* gnulib-local/lib/libxml/trionan.c: Replace it with Gnulib functions.
* gnulib-local/lib/libxml/trionan.h: Remove.
* gnulib-local/modules/libxml (Depends-on): Add 'isinf', 'isnan', and
'signbit'.
