Fix compilation error on MSVC due to conflict of different stdlib.h files.

Fix compilation error on MSVC due to conflict of different stdlib.h files.

Reported by Kirill Makurin <maiddaisuki@outlook.com> in
<https://lists.gnu.org/archive/html/bug-gettext/2025-05/msg00071.html>.

* autogen.sh: Change gnulib-tool invocation for libgrep.
* gettext-tools/configure.ac (AC_CONFIG_FILES): Add libgrep/gnulib-lib/Makefile.
* gettext-tools/libgrep/Makefile.am: Don't include Makefile.gnulib.
(SUBDIRS): New variable.
(noinst_LIBRARIES): Add libgrep.a.
(AM_CPPFLAGS): Add -I options for gnulib-lib subdirectory.
(libgrep_a_DEPENDENCIES): New variable.
* gettext-tools/libgrep/gnulib-lib/Makefile.am: New file.
* gettext-tools/src/Makefile.am (LIBGREP): Add ../libgrep/gnulib-lib/libgnu.a.