Ninja: Use absolute path to COMSPEC if possible

If the COMSPEC is set to an absolute path, use it instead of just
`cmd.exe`.  This avoids searching for the tool in the current working
directory and the `PATH`.  If COMSPEC is not set to an absolute path,
fall back to the existing behavior.

Issue: #18350

Source/cmGlobalNinjaGenerator.cxx

@@ -554,6 +554,15 @@ cmGlobalNinjaGenerator::cmGlobalNinjaGenerator(cmake* cm)
 {
 #ifdef _WIN32
   cm->GetState()->SetWindowsShell(true);
+
+  // Attempt to use full path to COMSPEC, default "cmd.exe"
+  std::string comspec;
+  if (cmSystemTools::GetEnv("COMSPEC", comspec) &&
+      cmSystemTools::FileIsFullPath(comspec)) {
+    this->Comspec = comspec;
+  } else {
+    this->Comspec = "cmd.exe";
+  }
 #endif
   this->FindMakeProgramFile = "CMakeNinjaFindMake.cmake";
 }

Source/cmGlobalNinjaGenerator.h

@@ -282,6 +282,10 @@ public:
   };
   MapToNinjaPathImpl MapToNinjaPath() { return { this }; }
 
+#ifdef _WIN32
+  std::string const& GetComspec() const { return this->Comspec; }
+#endif
+
   // -- Additional clean files
   void AddAdditionalCleanFile(std::string fileName, const std::string& config);
   const char* GetAdditionalCleanTargetName() const
@@ -592,6 +596,11 @@ private:
 
   codecvt_Encoding NinjaExpectedEncoding = codecvt_Encoding::None;
 
+#ifdef _WIN32
+  // Windows Command shell.
+  std::string Comspec;
+#endif
+
   bool DiagnosedCxxModuleNinjaSupport = false;
 
   void InitOutputPathPrefix();

Source/cmLocalNinjaGenerator.cxx

@@ -538,13 +538,14 @@ std::string cmLocalNinjaGenerator::BuildCommandLine(
 
   std::ostringstream cmd;
 #ifdef _WIN32
+  cmGlobalNinjaGenerator const* gg = this->GetGlobalNinjaGenerator();
   bool const needCMD =
     cmdLines.size() > 1 || (customStep.empty() && RuleNeedsCMD(cmdLines[0]));
   for (auto li = cmdLines.begin(); li != cmdLines.end(); ++li) {
     if (li != cmdLines.begin()) {
       cmd << " && ";
     } else if (needCMD) {
-      cmd << "cmd.exe /C \"";
+      cmd << gg->GetComspec() << " /C \"";
     }
     // Put current cmdLine in brackets if it contains "||" because it has
     // higher precedence than "&&" in cmd.exe