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.

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

@@ -53,9 +53,9 @@ class Flex(AutotoolsPackage):
     depends_on('automake', type='build', when='@:2.6.0')
     depends_on('libtool',  type='build', when='@:2.6.0')
 
-    # Build issue for v2.6.4 when gcc 7.2.0 is used.
+    # Build issue for v2.6.4 when gcc 7.2.0 is used on Ubuntu 17.10.
     # See issue #219; https://github.com/westes/flex/issues/219
-    conflicts('%gcc@7.2.0:', when='@2.6.4')
+    conflicts('%gcc@7.2.0: os=ubuntu17.10', when='@2.6.4')
 
     def url_for_version(self, version):
         url = "https://github.com/westes/flex"