phist can't be compiled with gcc older than 4.9.2 (#11677)

* phist can't be compiled with gcc older than 4.9.2

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

@@ -111,6 +111,10 @@ class Phist(CMakePackage):
     # required a separate package
     conflicts('+fortran', when='@:1.6.99')
 
+    # older gcc's may produce incorrect SIMD code and fail
+    # to compile some OpenMP statements
+    conflicts('%gcc@:4.9.1')
+
     def cmake_args(self):
         spec = self.spec