build: Fix race in post-install removal of *.a
There was an implicit dependency between the prerequisites of
install-exec-local, which may have caused error with make -jN.
Use install-exec-hook to remove the dependency.
Reported by Christian Weisgerber in:
<https://lists.gnu.org/archive/html/bug-gettext/2014-07/msg00008.html>.
* Makefile.am (install-exec-local): Remove.
(install-exec-hook): New rule, depend on install-exec-clean.
