for bootstrap" tweak to the lex Makefile to stop it building the library
too early.
This untangles things a bit more, it stops new bootstraps failing because
libl/libfl uses 'ld -O' before ld is updated.
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.
scripts check for. (when looking explicitly for flex..)
Also, do some man links for the commands under their alternate names.
Install the c++ FlexLexer.h file as per the "XXX TODO" comment.
