doc: Fix typo in "Installers" chapter
Add missing extern-inline.m4 into archive

Problem reported by Reuben Thomas in:
<https://lists.gnu.org/archive/html/bug-gettext/2013-12/msg00010.html>.
* gettext-runtime/m4/extern-inline.m4: New file, from gnulib.
* gettext-runtime/m4/intl.m4 (AM_INTL_SUBDIR): Require gl_EXTERN_INLINE.
* gettext-runtime/m4/Makefile.am (EXTRA_DIST): Update.
* gettext-tools/m4/Makefile.am (aclocal_DATA): Add extern-inline.m4.
* Makefile.am (distcheck-hook): Check extern-inline.m4.