Pacify "gcc -Wunused" for the input function from Flex.

Reported by Alex Rozenman.  This warning shows up with gcc-4.3.0
and later.
* src/scan-code.l: Add "%option noinput", which I cannot find in
the Flex manual, but which Flex has supported since at least as
far back as 2.5.4.  However, if any of our developers still use
Flex 2.5.4, they'll need to stop configuring with
--enable-gcc-warnings because "%option noinput" didn't work
correctly until Flex 2.5.6.
* src/scan-gram.l: Likewise.
* src/scan-skel.l: Likewise.

src/scan-code.l

@@ -17,7 +17,7 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
-%option debug nodefault nounput noyywrap never-interactive
+%option debug nodefault noinput nounput noyywrap never-interactive
 %option prefix="code_" outfile="lex.yy.c"
 
 %{

src/scan-gram.l

@@ -18,7 +18,7 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
-%option debug nodefault nounput noyywrap never-interactive
+%option debug nodefault noinput nounput noyywrap never-interactive
 %option prefix="gram_" outfile="lex.yy.c"
 
 %{

src/scan-skel.l

@@ -18,7 +18,7 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
-%option nodefault noyywrap nounput never-interactive debug
+%option nodefault noyywrap noinput nounput never-interactive debug
 %option prefix="skel_" outfile="lex.yy.c"
 
 %{