Reduce explicit use of __attribute__.

Reduce explicit use of __attribute__.

* autogen.sh (GNULIB_MODULES_RUNTIME_FOR_SRC, GNULIB_MODULES_TOOLS_FOR_SRC): Add noreturn.
* gettext-runtime/src/envsubst.c: Include noreturn.h.
(usage): Use _GL_NORETURN_FUNC.
* gettext-runtime/src/gettext.c: Likewise.
* gettext-runtime/src/ngettext.c: Likewise.
* gettext-tools/src/hostname.c: Likewise.
* gettext-tools/src/msgattrib.c: Likewise.
* gettext-tools/src/msgcat.c: Likewise.
* gettext-tools/src/msgcmp.c: Likewise.
* gettext-tools/src/msgcomm.c: Likewise.
* gettext-tools/src/msgconv.c: Likewise.
* gettext-tools/src/msgen.c: Likewise.
* gettext-tools/src/msgexec.c: Likewise.
* gettext-tools/src/msgfilter.c: Likewise.
* gettext-tools/src/msgfmt.c: Likewise.
* gettext-tools/src/msginit.c: Likewise.
* gettext-tools/src/msgmerge.c: Likewise.
* gettext-tools/src/msgunfmt.c: Likewise.
* gettext-tools/src/msguniq.c: Likewise.
* gettext-tools/src/recode-sr-latin.c: Likewise.
* gettext-tools/src/urlget.c: Likewise.
* gettext-tools/src/xgettext.c: Likewise.
* gettext-tools/src/msggrep.c: Include noreturn.h.
(no_pass, usage): Use _GL_NORETURN_FUNC.
* gettext-tools/src/msgl-iconv.c: Include noreturn.h.
(conversion_error): Use _GL_NORETURN_FUNC.
* gettext-tools/tests/tstgettext.c: Include noreturn.h.
(usage): Use _GL_NORETURN_FUNC.
* gettext-tools/tests/tstngettext.c: Likewise.