Make CMAKE_CXX_STANDARD default to 23 if not already specified

per Brad's suggestion.

Author: nat-goodspeed

indra/CMakeLists.txt

@@ -29,7 +29,9 @@ else()
   set( USE_AUTOBUILD_3P ON )
 endif()
 
-set(CMAKE_CXX_STANDARD 23)
+if (NOT DEFINED CMAKE_CXX_STANDARD)
+  set(CMAKE_CXX_STANDARD 23)
+endif()
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 include(Variables)