flex: use 2.6.3. by default (#8154)
flex: use 2.6.3. by default (#8154)

Fixes #8152

Flex 2.6.4 doesn't build with gcc@7:. This sets Flex 2.6.3 as the
preferred version. The flex package had a conflict directive for
this but originally the recorded conflict was specific to ubuntu;
this appears to be an issue on multiple operating systems so this
also updates the conflict to be more general and adds a comment
which links to the relevant issues.
mfherbst/spack,tmerrick1/spack,mfherbst/spack,matthiasdiener/spack,iulian787/spack,tmerrick1/spack,matthiasdiener/spack,iulian787/spack,LLNL/spack,krafczyk/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,matthiasdiener/spack,krafczyk/spack,mfherbst/spack,iulian787/spack,krafczyk/spack,krafczyk/spack,mfherbst/spack,tmerrick1/spack,mfherbst/spack,matthiasdiener/spack,LLNL/spack,matthiasdiener/spack,tmerrick1/spack,tmerrick1/spack,krafczyk/spack