Fix "warning: unannotated fall-through between switch labels".

Fix "warning: unannotated fall-through between switch labels".

* autogen.sh (GNULIB_MODULES_RUNTIME_FOR_SRC, GNULIB_MODULES_TOOLS_FOR_SRC,
GNULIB_MODULES_LIBGETTEXTPO): Add 'attribute'.
* libtextstyle/autogen.sh (GNULIB_MODULES): Likewise.
* gettext-runtime/intl/plural.y: Include attribute.h. Use FALLTHROUGH annotated
empty statements instead of /* FALLTHROUGH */ comments.
* gettext-runtime/src/gettext.c: Likewise.
* gettext-runtime/src/ngettext.c: Likewise.
* gettext-tools/src/format-boost.c: Likewise.
* gettext-tools/src/read-stringtable.c: Likewise.
* gettext-tools/src/write-csharp.c: Likewise.
* gettext-tools/src/write-java.c: Likewise.
* gettext-tools/src/write-po.c: Likewise.
* gettext-tools/src/x-awk.c: Likewise.
* gettext-tools/src/x-c.c: Likewise.
* gettext-tools/src/x-csharp.c: Likewise.
* gettext-tools/src/x-elisp.c: Likewise.
* gettext-tools/src/x-java.c: Likewise.
* gettext-tools/src/x-javascript.c: Likewise.
* gettext-tools/src/x-librep.c: Likewise.
* gettext-tools/src/x-lisp.c: Likewise.
* gettext-tools/src/x-perl.c: Likewise.
* gettext-tools/src/x-php.c: Likewise.
* gettext-tools/src/x-python.c: Likewise.
* gettext-tools/src/x-scheme.c: Likewise.
* gettext-tools/src/x-sh.c: Likewise.
* gettext-tools/src/x-smalltalk.c: Likewise.
* gettext-tools/src/x-tcl.c: Likewise.
* gettext-tools/src/x-vala.c: Likewise.
* gettext-tools/src/x-ycp.c: Likewise.
* gettext-tools/tests/tstgettext.c: Likewise.
* gettext-tools/tests/tstngettext.c: Likewise.
* libtextstyle/gnulib-local/lib/tparm.c: Likewise.
* gettext-runtime/src/escapes.h: Use FALLTHROUGH annotated empty statements
instead of /* FALLTHROUGH */ comments.