Optionally force conversion to relative path

In cmLocalGenerator::ConvertToRelativePath we normally convert to
relative path only if the local and remote paths both lie inside the
source tree or both lie inside the build tree.  This commit adds an
optional 'force' argument to allow conversion even when this rule is
violated.

Source/cmLocalGenerator.h

@@ -260,7 +260,7 @@ public:
    * or quoted.
    */
   std::string ConvertToRelativePath(const std::vector<std::string>& local,
-                                    const char* remote);
+                                    const char* remote, bool force=false);
 
   /**
    * Get the relative path from the generator output directory to a