Suppress "unused variable" warnings with older versions of flex.

Versions of flex before 2.5.36 might generate code that results in an
"unused variable" warning, when using %option reentrant.  Historically
we've worked around that by specifying -Wno-error, but that's an
unsatisfying solution.  The official "fix" for this was just to insert a
dummy reference to the variable, so write a small perl script that edits
the generated C code similarly.

The MSVC side of this is untested, but the buildfarm should soon reveal
if I broke that.

Discussion: https://postgr.es/m/25456.1487437842@sss.pgh.pa.us