VS10: Convert paths normally unless forced to relative

Most CMake generators produce relative paths within the build tree and
full paths to files outside the build tree.  Make VS 10 and VS 11
project files consistent with this approach except for paths forced to
be relative to work around a VS 10 bug.

Source/cmVisualStudio10TargetGenerator.h

@@ -50,6 +50,7 @@ private:
   };
   struct ToolSources: public std::vector<ToolSource> {};
 
+  std::string ConvertPath(std::string const& path, bool forceRelative);
   void ConvertToWindowsSlash(std::string& s);
   void WriteString(const char* line, int indentLevel);
   void WriteProjectConfigurations();