AM_GNU_GETTEXT: Define localedir_c and localedir_c_make.
* gettext-runtime/m4/gettext.m4 (AM_GNU_GETTEXT): Define and substitute
localedir_c and localedir_c_make.
* gettext-runtime/src/Makefile.am (AM_CPPFLAGS): Use $(localedir_c_make)
instead of $(localedir). This is useful in native Windows environments.
* gettext-runtime/intl/Makefile.am (AM_CPPFLAGS): Likewise.
* gettext-tools/src/Makefile.am (AM_CPPFLAGS): Likewise.
* gettext-tools/tests/Makefile.am (AM_CPPFLAGS): Likewise.
* gettext-tools/doc/gettext.texi (AM_GNU_GETTEXT): Small updates. Mention
localedir_c and localedir_c_make.
* NEWS: Mention the change.
