where to find c headers

gcc lcdtest.c -Isrc/ -L./libpifacecad/ -lpifacecad -L./libmcp23s17/ -lmcp23s17 -o lcdtest

fails into:
lcdtest.c:1:23: fatal error: pifacecad.h: No such file or directory
compilation terminated.

let’s google:

#include tells GCC to look where it finds the standard C headers, probably not where you have your header. #include “pifacecad.h tells GCC to look for headers in the current working directory.

#include =>
#include “pifacecad.h”

now gcc lcdtest.c -Isrc/ -L./libpifacecad/ -lpifacecad -L./libmcp23s17/ -lmcp23s17 -o lcdtest
works