bootstrap: prefer Clang over GNU on Darwin

bootstrap

@@ -860,7 +860,7 @@ cmake_toolchains=''
 case "${cmake_system}" in
   *AIX*)   cmake_toolchains='XL GNU' ;;
   *CYGWIN*) cmake_toolchains='GNU' ;;
-  *Darwin*) cmake_toolchains='GNU Clang' ;;
+  *Darwin*) cmake_toolchains='Clang GNU' ;;
   *Linux*) cmake_toolchains='GNU Clang XL PGI PathScale' ;;
   *MINGW*) cmake_toolchains='GNU' ;;
 esac