cxxmodules: Add more suggestions to no-modules-support diagnostics

Tell users what generators *do* support C++ modules.  Report the current
generator to make clear it is not one of those supporting modules.
Also clarify the purpose of the existing documentation references.

Source/cmGeneratorTarget.cxx

@@ -9227,11 +9227,15 @@ void cmGeneratorTarget::CheckCxxModuleStatus(std::string const& config) const
         cmGlobalGenerator::CxxModuleSupportQuery::Expected)) {
     this->Makefile->IssueMessage(
       MessageType::FATAL_ERROR,
-      cmStrCat(
-        "The target named \"", this->GetName(),
-        "\" has C++ sources that may use modules, but modules are not "
-        "supported by this generator.  See the cmake-cxxmodules(7) manual "
-        "and the CMAKE_CXX_SCAN_FOR_MODULES variable."));
+      cmStrCat("The target named \"", this->GetName(),
+               "\" has C++ sources that may use modules, but modules are not "
+               "supported by this generator:\n  ",
+               this->GetGlobalGenerator()->GetName(), '\n',
+               "Modules are supported only by Ninja, Ninja Multi-Config, "
+               "and Visual Studio generators for VS 17.4 and newer.  "
+               "See the cmake-cxxmodules(7) manual for details.  "
+               "Use the CMAKE_CXX_SCAN_FOR_MODULES variable to enable or "
+               "disable scanning."));
     return;
   }
 
@@ -9266,8 +9270,9 @@ void cmGeneratorTarget::CheckCxxModuleStatus(std::string const& config) const
         cmStrCat("The target named \"", this->GetName(),
                  "\" has C++ sources that may use modules, but the compiler "
                  "does not provide a way to discover the import graph "
-                 "dependencies.  See the cmake-cxxmodules(7) manual "
-                 "and the CMAKE_CXX_SCAN_FOR_MODULES variable."));
+                 "dependencies.  See the cmake-cxxmodules(7) manual for "
+                 "details.  Use the CMAKE_CXX_SCAN_FOR_MODULES variable to "
+                 "enable or disable scanning."));
     } break;
     case cmGeneratorTarget::Cxx20SupportLevel::Supported:
       // All is well.

Tests/RunCMake/CXXModules/NoCXX-stderr.txt

@@ -3,8 +3,15 @@
   language has not been enabled\.
 |CMake Error in CMakeLists.txt:
   The target named "nocxx" has C\+\+ sources that may use modules, but modules
-  are not supported by this generator\.  See the cmake-cxxmodules\(7\) manual
-  and the CMAKE_CXX_SCAN_FOR_MODULES variable\.
+  are not supported by this generator:
+
+    [^
+]+
+
+  Modules are supported only by Ninja, Ninja Multi-Config, and Visual Studio
+  generators for VS 17\.4 and newer\.  See the cmake-cxxmodules\(7\) manual for
+  details\.  Use the CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable
+  scanning\.
 |CMake Error in CMakeLists.txt:
   Target "nocxx" has source file
 

Tests/RunCMake/CXXModules/NoCXX20-stderr.txt

@@ -4,6 +4,13 @@
   "cxx_std_17"\.
 |CMake Error in CMakeLists.txt:
   The target named "nocxx20" has C\+\+ sources that may use modules, but
-  modules are not supported by this generator\.  See the cmake-cxxmodules\(7\)
-  manual and the CMAKE_CXX_SCAN_FOR_MODULES variable\.
+  modules are not supported by this generator:
+
+    [^
+]+
+
+  Modules are supported only by Ninja, Ninja Multi-Config, and Visual Studio
+  generators for VS 17\.4 and newer\.  See the cmake-cxxmodules\(7\) manual for
+  details\.  Use the CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable
+  scanning\.
 )

Tests/RunCMake/CXXModules/NoDyndepSupport-stderr.txt

@@ -7,6 +7,13 @@
   due to lack of required features\.  Ninja 1\.11 or higher is required\.
 |CMake Error in CMakeLists.txt:
   The target named "nodyndep" has C\+\+ sources that may use modules, but
-  modules are not supported by this generator\.  See the cmake-cxxmodules\(7\)
-  manual and the CMAKE_CXX_SCAN_FOR_MODULES variable\.
+  modules are not supported by this generator:
+
+    [^
+]+
+
+  Modules are supported only by Ninja, Ninja Multi-Config, and Visual Studio
+  generators for VS 17\.4 and newer\.  See the cmake-cxxmodules\(7\) manual for
+  details\.  Use the CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable
+  scanning\.
 )

Tests/RunCMake/CXXModules/NoScanningSourceFileProperty-stderr.txt

@@ -1,10 +1,17 @@
 (CMake Error in CMakeLists.txt:
   The target named "noscanning-sf-property" has C\+\+ sources that may use
   modules, but the compiler does not provide a way to discover the import
-  graph dependencies\.  See the cmake-cxxmodules\(7\) manual and the
-  CMAKE_CXX_SCAN_FOR_MODULES variable\.
+  graph dependencies\.  See the cmake-cxxmodules\(7\) manual for details\.  Use
+  the CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable scanning\.
 |CMake Error in CMakeLists.txt:
   The target named "noscanning-sf-property" has C\+\+ sources that may use
-  modules, but modules are not supported by this generator\.  See the
-  cmake-cxxmodules\(7\) manual and the CMAKE_CXX_SCAN_FOR_MODULES variable\.
+  modules, but modules are not supported by this generator:
+
+    [^
+]+
+
+  Modules are supported only by Ninja, Ninja Multi-Config, and Visual Studio
+  generators for VS 17\.4 and newer\.  See the cmake-cxxmodules\(7\) manual for
+  details\.  Use the CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable
+  scanning\.
 )

Tests/RunCMake/CXXModules/NoScanningTargetProperty-stderr.txt

@@ -1,10 +1,17 @@
 (CMake Error in CMakeLists.txt:
   The target named "noscanning-target-property" has C\+\+ sources that may use
   modules, but the compiler does not provide a way to discover the import
-  graph dependencies\.  See the cmake-cxxmodules\(7\) manual and the
-  CMAKE_CXX_SCAN_FOR_MODULES variable\.
+  graph dependencies\.  See the cmake-cxxmodules\(7\) manual for details\.  Use
+  the CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable scanning\.
 |CMake Error in CMakeLists.txt:
   The target named "noscanning-target-property" has C\+\+ sources that may use
-  modules, but modules are not supported by this generator\.  See the
-  cmake-cxxmodules\(7\) manual and the CMAKE_CXX_SCAN_FOR_MODULES variable\.
+  modules, but modules are not supported by this generator:
+
+    [^
+]+
+
+  Modules are supported only by Ninja, Ninja Multi-Config, and Visual Studio
+  generators for VS 17\.4 and newer\.  See the cmake-cxxmodules\(7\) manual for
+  details\.  Use the CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable
+  scanning\.
 )