> > could we check the version number instead? > > No, the header doesn't have version information.
So we need both a recent flex program and a recent flex header, right? For the former a version test would be easy, see http://git.savannah.nongnu.org/cgit/autoconf-archive.git/tree/m4/ax_prog_flex_version.m4 For the latter, I think your code completes the tests nicely. https://codereview.appspot.com/563560043/
