Revert "Build Flex-generated files with "warnings are errors"."

Revert "Build Flex-generated files with "warnings are errors"."

This reverts commit b56f53884be3bab935058b2bbbb4da0b8bbbe7f6.

Sadly, we *do* get warnings at this point with older versions of Flex,
such as the one on the 32-bit OS X buildbot.

Change-Id: I9aec1a16e9f2e1bbcfaac3dffdabdd89af5815e3
Reviewed-on: https://code.wireshark.org/review/12443
Reviewed-by: Guy Harris <guy@alum.mit.edu>