Fixes #24

I don't know if this the right fix, but I'm following in the makefile's
footsteps. Gcc complained about comparing signed and unsigned types, the
solution is to type cast it. At least, I think the solution is to
typecast it. Everything makes just fine after this commit.
Amend PATH so that OS X boxes with brew-installed flex 2.5.37 will use that.
Merge remote-tracking branch 'origin/fix_lexer_warnings'