flex: restrict GCC conflict to newer Ubuntu releases (#6963)
flex: restrict GCC conflict to newer Ubuntu releases (#6963)

flex 2.6.4 compiles fine with GCC 7 on (at least) Ubuntu 16.04 and Fedora 27.
matthiasdiener/spack,iulian787/spack,matthiasdiener/spack,iulian787/spack,iulian787/spack,tmerrick1/spack,krafczyk/spack,iulian787/spack,matthiasdiener/spack,EmreAtes/spack,mfherbst/spack,mfherbst/spack,EmreAtes/spack,LLNL/spack,krafczyk/spack,EmreAtes/spack,EmreAtes/spack,krafczyk/spack,LLNL/spack,LLNL/spack,EmreAtes/spack,tmerrick1/spack,mfherbst/spack,matthiasdiener/spack,krafczyk/spack,krafczyk/spack,LLNL/spack,tmerrick1/spack,LLNL/spack,iulian787/spack,tmerrick1/spack,mfherbst/spack,matthiasdiener/spack,mfherbst/spack,tmerrick1/spack