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

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