How to install libX11-dev on mac os x sierra

I am working on facial landmarks and therefore I have installed dlib on my mac. When I try to execute a program in Xcode from learnopencv.com, the compiler says:

error "DLIB_NO_GUI_SUPPORT is defined so you can't use the GUI code. Turn DLIB_NO_GUI_SUPPORT off if you want to use it."
error "Also make sure you have libx11-dev installed on your system"

I already installed XQuartz. I don't know what to do anymore. Can anyone help me out here?

How do I know if x11 is installed on my Mac?

A quick way for them to check is to type startX at the command line and see if X11 starts up (a dock icon with a black X should appear). BUT 90% of Mac OS X users have never used the command line... Maybe it is better to tell them to look for /Applications/Utilities/X11 to see if it is installed.

I suffered from this error for 3 days and finally fixed with: