intl: Consolidate fallback _GL_* macro definitions into intl.m4

intl: Consolidate fallback _GL_* macro definitions into intl.m4

We had defined _GL_* macros used by libintl itself in the corresponding
C files.  However, it will increase divergence between Gnulib and gettext.
This patch moves the fallback macro definitions into M4 and emit them only
when the project is not using Gnulib.

* gettext-runtime/m4/intl.m4 (gt_GL_ATTRIBUTE): New macro.
(AM_INTL_SUBDIR): Require gt_GL_ATTRIBUTE.
* gettext-runtime/intl/localename.c: Update from Gnulib.
2014-07-10  Assaf Gordon  <assafgordon@gmail.com>
    localename: avoid -Wsuggest-attribute={const,pure} warnings
    * lib/localename.c (string_hash): Tag internal function as pure.
* gettext-runtime/intl/xsize.c (XSIZE_INLINE): Remove _GL_UNUSED definition.
It will be set through intl.m4.
* Makefile.am (distcheck-hook): Add back localename.c check.