-xM

Runs only the macro preprocessor on the named C programs, requesting that it generate makefile dependencies and send the result to the standard output (see make(1) for details about makefiles and dependencies).

For example:

#include <unistd.h>
void main(void)
{}

generates this output:

e.o: e.c
e.o: /usr/include/unistd.h
e.o: /usr/include/sys/types.h
e.o: /usr/include/sys/machtypes.h
e.o: /usr/include/sys/select.h
e.o: /usr/include/sys/time.h
e.o: /usr/include/sys/types.h
e.o: /usr/include/sys/time.h
e.o: /usr/include/sys/unistd.h