build: Support "make dist" without prior "make".
This is required by the GNU Coding Standards
<https://www.gnu.org/prep/standards/html_node/Standard-Targets.html>.
Reported by <nroycea+gnu@gmail.com> in
<https://lists.gnu.org/archive/html/bug-gettext/2024-07/msg00010.html>.
* Makefile.am (distdir1): New target.
* configure.ac: Apply FIX_MAKEFILE_DISTRIB on Makefile.
* gettext-runtime/Makefile.am (distdir1, gen-man1): New targets.
* gettext-runtime/configure.ac: Apply FIX_MAKEFILE_DISTRIB on Makefile.
* gettext-tools/Makefile.am (programs-for-distdir, distdir1, gen-man1,
distdir2): New targets.
* gettext-tools/configure.ac: Apply FIX_MAKEFILE_DISTRIB on Makefile.
* gettext-tools/src/Makefile.am (built-sources): New target.
