Fix regression to rc.exe usage.

(Yes, it parses includes different from cl.exe, somehow...)

ambuild2/frontend/v2_2/cpp/builders.py

@@ -283,7 +283,7 @@ def buildRcItem(self, inputObj, sourceFile, encodedName):
         for include in (self.compiler.includes + self.compiler.cxxincludes):
             if isinstance(include, PchNodes):
                 continue
-            rc_argv += ['/i', self.vendor.IncludePath(objectFile, include)]
+            rc_argv += ['/i', self.vendor.RcIncludePath(objectFile, include)]
         rc_argv += ['/fo' + objectFile, sourceFile]
 
         return RCFile(self, inputObj, [objectFile, encodedName + '.i'], cl_argv, rc_argv)

ambuild2/frontend/v2_2/cpp/msvc.py

@@ -108,6 +108,18 @@ def IncludePath(outputPath, includePath):
             return includePath
         return os.path.relpath(includePath, outputPath)
 
+    @staticmethod
+    def RcIncludePath(outputPath, includePath):
+        # Same as IncludePath, but prefer absolute paths because it breaks on
+        # relative ones...
+        outputPath = os.path.normcase(outputPath)
+        includePath = os.path.normcase(includePath)
+        outputDrive = os.path.splitdrive(outputPath)[0]
+        includeDrive = os.path.splitdrive(includePath)[0]
+        if outputDrive != includeDrive:
+            return includePath
+        return os.path.relpath(includePath, outputPath)
+
     def formatInclude(self, output_path, include):
         return ['/I', MSVC.IncludePath(output_path, include)]