I'm trying to run a program from the terminal on my mac, but I'm getting an error on a line

mpicc -c -g pumax.c
pumax.c:29:10: fatal error: 'X11/Xlib.h' file not found
   29 | #include 
      |          ^~~~~~~~~~~~
1 error generated.
make: *** [pumax.o] Error 1

I have X11 installed as /opt/X11 but it contains lib, not Xlib.h. Typing port installed to list my MacPorts installed packages, I see xorg-libX11 @1.8.12_0 (active), which is the developer version of X11 (?) so it should have Xlib.h right?

I can my issue is very similar to this post During make mupdf, fatal error: X11/Xlib.h file not found in mac. I've tried the symlink solution but it hasn't worked unfortunately. I've also reinstalled XQuartz and that's not helped.

Any help would be really well appreciated, I'm very new to this and I did my best trying to research the problem but I've come up with nothing.