g++ -H ...
will also print the full path of include files in a format which shows which header includes which
This will give make dependencies which list absolute paths of include files:
gcc -M showtime.c
If you don't want the system includes (i.e. #include <something.h>) then use:
#include <something.h>
gcc -MM showtime.c
Sure use
g++ -E -dI ... (whatever the original command arguments were)