build: Simplify.
This partially reverts the commit 2020-12-28 "Update after gnulib changed:
Fix link errors on native Windows."
* autogen.sh (GNULIB_MODULES_TOOLS_FOR_SRC, GNULIB_MODULES_LIBGETTEXTPO): Remove
strdup-posix.
* gettext-tools/libgettextpo/Makefile.am (config.h): Don't define GNULIB_GETCWD.
