Allow EAMxx to take a cxx standard option

Author: tcclevenger

components/eamxx/CMakeLists.txt

@@ -51,8 +51,10 @@ if (SCREAM_CIME_BUILD)
     ${CMAKE_CURRENT_SOURCE_DIR}/cmake/cime)
 endif ()
 
-# We want to use C++17 in EAMxx
-set(CMAKE_CXX_STANDARD 17)
+if (NOT CMAKE_CXX_STANDARD)
+  # Default to C++17 in EAMxx
+  set(CMAKE_CXX_STANDARD 17)
+endif()
 
 if (NOT SCREAM_CIME_BUILD)
   project(SCREAM CXX C Fortran)