Escape the source file to be compiled if required.

Whitespaces in paths can otherwise cause invalid command lines to
be generated.

Source/cmNinjaTargetGenerator.cxx

@@ -492,7 +492,10 @@ cmNinjaTargetGenerator
     cmLocalGenerator::RuleVariables compileObjectVars;
     std::string lang = language;
     compileObjectVars.Language = lang.c_str();
-    compileObjectVars.Source = sourceFileName.c_str();
+    std::string escapedSourceFileName =
+      this->LocalGenerator->ConvertToOutputFormat(
+        sourceFileName.c_str(), cmLocalGenerator::SHELL);
+    compileObjectVars.Source = escapedSourceFileName.c_str();
     compileObjectVars.Object = objectFileName.c_str();
     compileObjectVars.Flags = vars["FLAGS"].c_str();
     compileObjectVars.Defines = vars["DEFINES"].c_str();