build: Work around undefined _GL_ATTRIBUTE_PURE in libintl
* Makefile.am (distcheck-hook): Ignore the difference between our
localename.c and the latest Gnulib's, which utilizes
_GL_ATTRIBUTE_PURE not available from libintl. This is a
temporary fix for the 0.19.2 release and should be reverted once
it is fixed either in gettext or Gnulib.
