Address legion package build issue on macOS with apple-clang-12 (#28241)

* Added C++ standard variant as a primary work-around for issues with apple-clang 12.x
but could also be helpful for broader uses.

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

@@ -62,6 +62,13 @@ class Legion(CMakePackage):
     depends_on('papi', when='+papi')
     depends_on('zlib', when='+zlib')
 
+    # A C++ standard variant to work-around some odd behaviors with apple-clang
+    # but this might be helpful for other use cases down the road.  Legion's
+    # current development policy is C++11 or greater so we capture that aspect
+    # here.
+    cpp_stds = ["11", "14", "17", "20"]
+    variant('cxxstd', default="11", values=cpp_stds, multi=False)
+
     # TODO: Need a AMD/HIP variant to match support landing in 21.03.0.
 
     # Network transport layer: the underlying data transport API should be used for
@@ -201,7 +208,10 @@ def validate_gasnet_root(value):
     def cmake_args(self):
         spec = self.spec
         cmake_cxx_flags = []
-        options = []
+        from_variant = self.define_from_variant
+        options = [
+            from_variant("CMAKE_CXX_STANDARD", "cxxstd")
+        ]
 
         if 'network=gasnet' in spec:
             options.append('-DLegion_NETWORKS=gasnetex')