cm::filesystem: Restore detection of std::filesystem support on Windows

Since commit b86a77e8c8 (cm::filesystem::path: ensure independence from
the program locale., 2026-01-05, v4.3.0-rc1~185^2) detection always
fails due on Windows to using `fs` instead of `std::filesystem`.

Also remove the `weakly_canonical` check I added in commit ad4481fa00
(Source: Enhance check for C++17 filesystem support, 2023-12-02,
v3.29.0-rc1~296^2).  It was a bad idea since it accesses the filesystem,
and so in general this can give false negative say in the case of
`CMAKE_BINARY_DIR` being on a symlinked drive.

Fixes: #28221

Source/Checks/cm_cxx_filesystem.cxx

@@ -26,20 +26,14 @@ int main()
     return 1;
   }
 
-  std::filesystem::path b1("C:\\path\\y\\..\\");
-  if (std::filesystem::weakly_canonical("\\\\?\\C:\\path\\x\\..") !=
-      b1.lexically_normal()) {
-    return 1;
-  }
-
   // LWG 3070 not implemented on some old Visual Studio compilers (2017, for
-  // example)
+  // example), or on MinGW Clang.
   if (!std::filesystem::path("/a:/b:").lexically_relative("/a:/c:").empty()) {
     return 1;
   }
   // LWG 3096 not implemented on some old Visual Studio compilers (2017, for
   // example)
-  if (fs::path("/a").lexically_relative("/a/.") != ".") {
+  if (std::filesystem::path("/a").lexically_relative("/a/.") != ".") {
     return 1;
   }
 #endif