GCC on Mojave: The compiler cannot read the includes at -I /opt/X11/include/X11

Trying to compile with GCC on Mojave and am getting an error.

gcc  -g -ansi -c test_scroll_images_3x.c -I /opt/X11/include/X11

test_scroll_images_3x.c:17:11: fatal error: 'X11/Xlib.h' file not found# include <X11/Xlib.h>          ^~~~~~~~~~~~1 error generated.

I have several subroutines like this, and all have the same problem.