Here's a rough overview of how to port d2c. Take this with a grain of salt; the rules change occasionally.
Add your platform to the platforms.descr file and configure.in. This will require a bit of delibrate research.
Look in other places, such as the various Makegen files and platform-specific Melange code. Do something intelligent.
Make sure that the Boehm GC runs on your platform. If not, you need to talk to the Boehm list.
Run configure with the flag --enable-mindy-bootstrap, then run make. Be prepared to wait, and be ready to fix any bugs you encounter.
<firstname.lastname@example.org> when it
breaks. We'll help you sort out the mess.
The more your platform looks like a 32-bit Unix system with the GNU tools, the easier things will be. Supporting 64-bit platforms is a minor project requiring a number of changes to d2c (Mindy already works on Alpha Linux, thanks to the efforts of Jeff Dubrule). Windows support has been broken slightly since the 2.0 days, but most of the problems are in the build system, not the source itself.
Macintoshes, vendor C compilers and other such oddities are the responsibility of those who use them. We'll be happy to help, but you'll probably need to find a significant number of dedicated volunteers to help you fix any problems you encounter.