dealii: use cmake option to set c++ standard (spack#1522)

* use cmake option to set c++ standard

* use define_from_variant

Author: dabele

repos/spack_repo/builtin/packages/dealii/package.py

@@ -498,8 +498,7 @@ def cmake_args(self):
 
         # Enforce the specified C++ standard
         if spec.variants["cxxstd"].value != "default":
-            cxxstd = spec.variants["cxxstd"].value
-            cxx_flags.extend(["-std=c++{0}".format(cxxstd)])
+            options.append(self.define_from_variant("CMAKE_CXX_STANDARD", "cxxstd"))
 
         # Performance
         # Set recommended flags for maximum (matrix-free) performance, see