but flex still generates "#include <FlexLexer.h". As a result,
C++ sources flex generates failed to be compiled.
PR: 7575
Reviewed by: phk
Submitted by: MOROHOSHI Akihiko <moro@remus.dti.ne.jp>
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.
