CompileFeatures: memoize C++ compilers with full language level support

Previously compilers that had full support for a language standard level
were still verified every time a new build directory was created.  Now
we record this information and insert the correct granular compile
features instead of doing a `try_compile`.

Modules/Compiler/CMakeCommonCompilerMacros.cmake

@@ -94,18 +94,43 @@ endmacro()
 macro(cmake_record_cxx_compile_features)
   set(_result 0)
   if(_result EQUAL 0 AND DEFINED CMAKE_CXX20_STANDARD_COMPILE_OPTION)
-    _record_compiler_features_cxx(20)
+    if(CMAKE_CXX20_STANDARD__HAS_FULL_SUPPORT)
+      _has_compiler_features_cxx(20)
+    else()
+      _record_compiler_features_cxx(20)
+    endif()
+    unset(CMAKE_CXX20_STANDARD__HAS_FULL_SUPPORT)
   endif()
   if(_result EQUAL 0 AND DEFINED CMAKE_CXX17_STANDARD_COMPILE_OPTION)
-    _record_compiler_features_cxx(17)
+    if(CMAKE_CXX17_STANDARD__HAS_FULL_SUPPORT)
+      _has_compiler_features_cxx(17)
+    else()
+      _record_compiler_features_cxx(17)
+    endif()
+    unset(CMAKE_CXX17_STANDARD__HAS_FULL_SUPPORT)
   endif()
   if(_result EQUAL 0 AND DEFINED CMAKE_CXX14_STANDARD_COMPILE_OPTION)
-    _record_compiler_features_cxx(14)
+    if(CMAKE_CXX14_STANDARD__HAS_FULL_SUPPORT)
+      _has_compiler_features_cxx(14)
+    else()
+      _record_compiler_features_cxx(14)
+    endif()
+    unset(CMAKE_CXX14_STANDARD__HAS_FULL_SUPPORT)
   endif()
   if(_result EQUAL 0 AND DEFINED CMAKE_CXX11_STANDARD_COMPILE_OPTION)
-    _record_compiler_features_cxx(11)
+    if(CMAKE_CXX11_STANDARD__HAS_FULL_SUPPORT)
+      _has_compiler_features_cxx(11)
+    else()
+      _record_compiler_features_cxx(11)
+    endif()
+    unset(CMAKE_CXX11_STANDARD__HAS_FULL_SUPPORT)
   endif()
   if(_result EQUAL 0 AND DEFINED CMAKE_CXX98_STANDARD_COMPILE_OPTION)
-    _record_compiler_features_cxx(98)
+    if(CMAKE_CXX98_STANDARD__HAS_FULL_SUPPORT)
+      _has_compiler_features_cxx(98)
+    else()
+      _record_compiler_features_cxx(98)
+    endif()
+    unset(CMAKE_CXX98_STANDARD__HAS_FULL_SUPPORT)
   endif()
 endmacro()

Modules/Internal/FeatureTesting.cmake

@@ -99,3 +99,7 @@ macro(_has_compiler_features_c std)
   list(APPEND CMAKE_C${std}_COMPILE_FEATURES c_std_${std})
   _has_compiler_features(C ${std} "${CMAKE_C${std}_STANDARD_COMPILE_OPTION}" CMAKE_C${std}_COMPILE_FEATURES)
 endmacro()
+macro(_has_compiler_features_cxx std)
+  list(APPEND CMAKE_CXX${std}_COMPILE_FEATURES cxx_std_${std})
+  _has_compiler_features(CXX ${std} "${CMAKE_CXX${std}_STANDARD_COMPILE_OPTION}" CMAKE_CXX${std}_COMPILE_FEATURES)
+endmacro()