Autogen: Fallback on internal qrc parser when RCC isn't built yet

When building a Qt project, the AUTORCC functionality, by default,
uses the rcc binary to get the contents of a qrc file for
dependency information. This is done at CMake "generate" time.

The problem is that while configuring Qt itself, the rcc binary is
not built yet.
In that case, to get the contents of the qrc file, fall back to
the code branch which uses an ifstream instead of the rcc binary.

Source/cmQtAutoGenInitializer.cxx

@@ -594,7 +594,7 @@ bool cmQtAutoGenInitializer::InitRcc()
     if (this->QtVersion.Major == 5 || this->QtVersion.Major == 6) {
       if (stdOut.find("--list") != std::string::npos) {
         this->Rcc.ListOptions.emplace_back("--list");
-      } else {
+      } else if (stdOut.find("-list") != std::string::npos) {
         this->Rcc.ListOptions.emplace_back("-list");
       }
     }
@@ -1581,6 +1581,10 @@ bool cmQtAutoGenInitializer::GetQtExecutable(GenVarsT& genVars,
         print_err(prop + " evaluates to an empty value");
         return false;
       }
+
+      // Check if the provided executable already exists (it's possible for it
+      // not to exist when building Qt itself).
+      genVars.ExecutableExists = cmSystemTools::FileExists(genVars.Executable);
       return true;
     }
   }
@@ -1630,6 +1634,7 @@ bool cmQtAutoGenInitializer::GetQtExecutable(GenVarsT& genVars,
       print_err(err);
       return false;
     }
+    genVars.ExecutableExists = true;
   }
 
   return true;
@@ -1648,7 +1653,7 @@ bool cmQtAutoGenInitializer::RccListInputs(std::string const& fileName,
     error += "\n";
     return false;
   }
-  if (!this->Rcc.ListOptions.empty()) {
+  if (this->Rcc.ExecutableExists && !this->Rcc.ListOptions.empty()) {
     // Use rcc for file listing
     if (this->Rcc.Executable.empty()) {
       error = "rcc executable not available";

Source/cmQtAutoGenInitializer.h

@@ -68,6 +68,7 @@ public:
     std::string ExecutableTargetName;
     cmGeneratorTarget* ExecutableTarget = nullptr;
     std::string Executable;
+    bool ExecutableExists = false;
 
     /// @brief Constructor
     GenVarsT(GenT gen, std::string const& genName,