build: Simplify further.
* gettext-runtime/intl/Makefile.am: Assume BUILD_INCLUDED_LIBINTL is true.
* gettext-runtime/intl/configure.ac: Remove BUILD_INCLUDED_LIBINTL conditional.
* gettext-runtime/m4/gettext.m4 (AM_GNU_GETTEXT): Don't set
BUILD_INCLUDED_LIBINTL.
