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

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.