build: do not prototype flex-generated functions.

Some versions of Flex, possibly modified by the distribution package
maintainers, have incompatible signatures.  Since newer versions of
Flex prototype their functions, avoid the conflicts in that case.
Reported by Stefano Lattarini.
<http://lists.gnu.org/archive/html/bug-bison/2012-05/msg00012.html>.

* src/flex-scanner.h (FLEX_VERSION_GT): New.
Use it to issue prototypes for flex-generated functions only for
versions up to 2.5.31, in accordance with the comment.
See commit dc9701e848f27ae64b6ddcf809580998667d60f2.
Use it to define yylex_destroy when needed.