Merge branch 'tcclevenger/eamxx/updates_needed_for_cpp20' into next (PR #7691)

In C++20 you cannot aggregate initialize a stuct which has a default
constructor defined. We shouldn't need these constructors since views
themselves have default constructors.

Also, allow EAMxx to take a CMAKE_CXX_STANDARD option (default to C++17).

Creating this PR so that #7643 won't have to run full CI while I test.

[BFB]

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)