flex: Apply 2.6.4 patch regardless of gcc version (#10841)
flex: Apply 2.6.4 patch regardless of gcc version (#10841)

It seems that this is actually a glibc problem and while 2.6.4 builds
without the patch on newer versions of gcc (@8:), it still sometimes
segfaults (as observed during the doxygen build).
LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack