libgettextpo: Don't rely on the undocumented variable libgettextpo_program_name.
* gnulib-local/lib/xerror.c: If IN_LIBGETTEXTPO is defined, include
getprogname.h instead of progname.h and define program_name as a macro.
* gnulib-local/lib/error-progname.c: Likewise.
* gettext-tools/src/po-xerror.c: Likewise.
* autogen.sh (GNULIB_MODULES_LIBGETTEXTPO): Remove progname.
Pass --avoid=progname to gnulib-tool invocation for libgettextpo.
