If you are on Linux / Unix, building the GPAC utilities is a really simple process.
Get the latest stable release from here, and then do the usual ./configure, make and make install.
For Mac OS users there is an extra step that you have to do if you want to be able to build and install the GPAC utilities without headaches, and it involves the editing of a source file in the package.
So, after you have downloaded the sources, open the file src/utils/os_net.c , line 78 reads:
#ifndef __DARWIN__right AFTER it, add the following:
typedef unsigned long u_long;save the file, and you can now configure, build and install the GPAC utilities (MP4Box) on Mac OS without any problem.