gettext-runtime: Provide helper script to build against uninstalled binaries.
Prompted by <https://gcc.gnu.org/pipermail/gcc/2022-June/238929.html>.
* gettext-runtime/uninstalled-config.sh.in: New file.
* gettext-runtime/configure.ac: New variable ENABLE_SHARED. Generate
uninstalled-config.sh at configure time.