Remove use of Runtime.GetSystemVersion

This API is not available in portable CoreFx.  Fall back to our original
behavior of simply specifying the runtime.  The one exception is in
internal builds where we must respect the COMPLUS flags.

closes #1961

src/Compilers/Core/Desktop/Interop/ClrStrongName.cs

@@ -23,8 +23,24 @@ internal static IClrStrongName GetInstance()
             var clrStrongNameGuid = new Guid(0x9FD93CCF, 0x3280, 0x4391, 0xB3, 0xA9, 0x96, 0xE1, 0xCD, 0xE7, 0x7C, 0x8D);
 
             var metaHost = (IClrMetaHost)nCreateInterface(metaHostClsid, metaHostGuid);
-            var runtime = (IClrRuntimeInfo)metaHost.GetRuntime(RuntimeEnvironment.GetSystemVersion(), clrRuntimeInfoGuid);
+            var runtime = (IClrRuntimeInfo)metaHost.GetRuntime(GetRuntimeVersion(), clrRuntimeInfoGuid);
             return (IClrStrongName)runtime.GetInterface(clrStrongNameClsid, clrStrongNameGuid);
         }
+
+        internal static string GetRuntimeVersion()
+        {
+            // When running in a complus environment we must respect the specified CLR version.  This 
+            // important to keeping internal builds runnning. 
+            if (!string.IsNullOrEmpty(Environment.GetEnvironmentVariable("COMPLUS_InstallRoot")))
+            {
+                var version = Environment.GetEnvironmentVariable("COMPLUS_Version");
+                if (!string.IsNullOrEmpty(version))
+                {
+                    return version;
+                }
+            }
+
+            return "v4.0.30319";
+        }
     }
 }