BUG: Fix try_compile during EnableLanguage

  - The source-file signature of try_compile looks up the language
    of the source file using the extension-to-language map so that
    it knows what language to enable in the generated project.
  - This map needs to be filled before loading a file specified by
      CMAKE_USER_MAKE_RULES_OVERRIDE
      CMAKE_USER_MAKE_RULES_OVERRIDE_<LANG>
    so that the user file may call the try_compile() source-file
    signature.
  - It must still be re-filled after loading CMake<LANG>Information.cmake
    in case the compiler- or platform-specific files added anything.
  - See bug #7340.

Source/cmGlobalGenerator.cxx

@@ -559,6 +559,11 @@ for checking whether the language is already enabled. After setting this
 flag still the values from the cmake variables have to be copied into the
 internal maps, this is done in SetLanguageEnabledMaps() which is called
 after the system- and compiler specific files have been loaded.
+
+This split was done originally so that compiler-specific configuration
+files could change the object file extension
+(CMAKE_<LANG>_OUTPUT_EXTENSION) before the CMake variables were copied
+to the C++ maps.
 */
 void cmGlobalGenerator::SetLanguageEnabled(const char* l, cmMakefile* mf)
 {
@@ -566,9 +571,16 @@ void cmGlobalGenerator::SetLanguageEnabled(const char* l, cmMakefile* mf)
   this->SetLanguageEnabledMaps(l, mf);
 }
 
-void cmGlobalGenerator::SetLanguageEnabledFlag(const char* l, cmMakefile*)
+void cmGlobalGenerator::SetLanguageEnabledFlag(const char* l, cmMakefile* mf)
 {
   this->LanguageEnabled[l] = true;
+
+  // Fill the language-to-extension map with the current variable
+  // settings to make sure it is available for the try_compile()
+  // command source file signature.  In SetLanguageEnabledMaps this
+  // will be done again to account for any compiler- or
+  // platform-specific entries.
+  this->FillExtensionToLanguageMap(l, mf);
 }
 
 void cmGlobalGenerator::SetLanguageEnabledMaps(const char* l, cmMakefile* mf)
@@ -627,26 +639,37 @@ void cmGlobalGenerator::SetLanguageEnabledMaps(const char* l, cmMakefile* mf)
       }
     }
 
-  std::string extensionsVar = std::string("CMAKE_") +
-    std::string(l) + std::string("_SOURCE_FILE_EXTENSIONS");
+  // The map was originally filled by SetLanguageEnabledFlag, but
+  // since then the compiler- and platform-specific files have been
+  // loaded which might have added more entries.
+  this->FillExtensionToLanguageMap(l, mf);
+
   std::string ignoreExtensionsVar = std::string("CMAKE_") +
     std::string(l) + std::string("_IGNORE_EXTENSIONS");
   std::string ignoreExts = mf->GetSafeDefinition(ignoreExtensionsVar.c_str());
-  std::string exts = mf->GetSafeDefinition(extensionsVar.c_str());
   std::vector<std::string> extensionList;
-  cmSystemTools::ExpandListArgument(exts, extensionList);
+  cmSystemTools::ExpandListArgument(ignoreExts, extensionList);
   for(std::vector<std::string>::iterator i = extensionList.begin();
       i != extensionList.end(); ++i)
     {
-    this->ExtensionToLanguage[*i] = l;
+    this->IgnoreExtensions[*i] = true;
     }
-  cmSystemTools::ExpandListArgument(ignoreExts, extensionList);
+
+}
+
+void cmGlobalGenerator::FillExtensionToLanguageMap(const char* l,
+                                                   cmMakefile* mf)
+{
+  std::string extensionsVar = std::string("CMAKE_") +
+    std::string(l) + std::string("_SOURCE_FILE_EXTENSIONS");
+  std::string exts = mf->GetSafeDefinition(extensionsVar.c_str());
+  std::vector<std::string> extensionList;
+  cmSystemTools::ExpandListArgument(exts, extensionList);
   for(std::vector<std::string>::iterator i = extensionList.begin();
       i != extensionList.end(); ++i)
     {
-    this->IgnoreExtensions[*i] = true;
+    this->ExtensionToLanguage[*i] = l;
     }
-
 }
 
 bool cmGlobalGenerator::IgnoreFile(const char* l)

Source/cmGlobalGenerator.h

@@ -264,6 +264,7 @@ protected:
                         projectTargets);
   void SetLanguageEnabledFlag(const char* l, cmMakefile* mf);
   void SetLanguageEnabledMaps(const char* l, cmMakefile* mf);
+  void FillExtensionToLanguageMap(const char* l, cmMakefile* mf);
 
   virtual bool CheckALLOW_DUPLICATE_CUSTOM_TARGETS();