Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Connect flex 2.5.37 to the build and bump __FreeBSD_version.

$Header$ -> $FreeBSD$

Finish the import and merge in FreeBSD specific changes.

Add a 'bootstrap' target which *must* be run before building the new
version, since the new scanner relies on the current version of flex to
build itself otherwise.

This commit was generated by cvs2svn to compensate for changes in r16514,
which included commits to RCS files with non-trunk default branches.

Flex version 2.5.3 from Vern Paxson at LBL.