root: Record that old versions of ROOT don't support modern GCC (#14250)

* Record that old versions of ROOT don't support modern GCC

* Well, actually I don't know about 6.07

* Fix typo and follow odd version recommendation from @chissg

var/spack/repos/builtin/packages/root/package.py

@@ -234,6 +234,9 @@ class Root(CMakePackage):
     # See https://sft.its.cern.ch/jira/browse/ROOT-7517
     conflicts('%intel')
 
+    # ROOT <6.08 was incompatible with the GCC 5+ ABI
+    conflicts('%gcc@5.0.0:', when='@:6.07.99')
+
     # See README.md
     conflicts('+http',
               msg='HTTP server currently unsupported due to dependency issues')