new directive: conflicts() (#3125)

* Add conflicts(<spec>) directive
* openblas: added conflicts for intel@16 refs #3119
* added brief docs and unit tests

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

@@ -59,6 +59,8 @@ class Openblas(MakefilePackage):
 
     parallel = False
 
+    conflicts('%intel@16', when='@0.2.15:0.2.19')
+
     @run_before('edit')
     def check_compilers(self):
         # As of 06/2016 there is no mechanism to specify that packages which