Mark a compiler/version conflict

With Intel 14.0.4 on Linux for icu4c 60.1 and higher:
```
locid.cpp(1156): error spack#1140: a using-declaration may not name a constructor or destructor
        using KeywordEnumeration::KeywordEnumeration;
```

var/spack/repos/builtin/packages/icu4c/package.py

@@ -28,6 +28,7 @@ class Icu4c(AutotoolsPackage):
             description='Use the specified C++ standard when building')
 
     depends_on('python', type='build', when='@64.1:')
+    conflicts('%intel@:15', when='@60.1:')
 
     configure_directory = 'source'