build: Fix a build issue on AIX.

build: Fix a build issue on AIX.

How to reproduce:
$ cd gettext-20231111
$ ~/build-64-gcc --disable-shared
$ (cd build-64-gcc && gmake install)
$ cd ..
$ cd gettext-0.22.3
$ ~/build-64-gcc
=> Fails with "ERROR: Undefined symbol: .syntax_check_message_list"
because the link uses $PREFIX/lib/libgettextsrc.a, which is a non-shared
library that does not export 'syntax_check_message_list'.

* gettext-tools/configure.ac (INSTALL_PRIVATE_LIBRARIES): New conditional.
* gettext-tools/src/Makefile.am (noinst_LTLIBRARIES): Define as alternative to
lib_LTLIBRARIES.
(install-exec-clean): Remove rule.
* gettext-tools/gnulib-lib/Makefile.am (lib_LTLIBRARIES): Initialize.
* gnulib-local/modules/gettext-tools-misc (Makefile.am): Define
noinst_LTLIBRARIES as alternative to lib_LTLIBRARIES. Remove
'install-exec-clean' rule.