We are developing on a machine with flex 2.6.4 and we couldn't build.
This was fixed in 72b1e3a2 in upstream (9.6 at the time) in 2016, but
wasn't backported to 9.1 or older.

Original commit message:
> Now that we know about the %top{} trick, we can revert to building flex
> lexers as separate .o files.  This is worth doing for a couple of reasons
> besides sheer cleanliness.  We can narrow the scope of the -Wno-error flag
> that's forced on scan.c.  Also, since these grammar and lexer files are
> so large, splitting them into separate build targets should have some
> advantages in build speed, particularly in parallel or ccache'd builds.
>
> We have quite a few other .l files that could be changed likewise, but the
> above arguments don't apply to them, so the benefit of fixing them seems
> pretty minimal.  Leave the rest for some other day.

(cherry picked from commit 72b1e3a2)

Fixes greenplum-db/gpdb#4863
Co-authored-by: NJesse Zhang <sbjesse@gmail.com>
Co-authored-by: NAsim R P <apraveen@pivotal.io>