XORP is a large application, but it is intended to be portable. We've tried as far as possible to keep the C++ code close to revision 3 of the C++ standard (ISO14882). The only compilers tested to date have been different versions of GNU g++. The code compiles on all versions we've tested from 2.95 upwards.
A design goal for XORP is placing all system dependencies in the Forwarding Engine Abstraction (FEA) process. To make XORP run on a new platform the FEA must be modified to support it. The bottom end of the FEA talks to the operating system to install routes and use raw sockets. In most cases, it is this bottom end that needs writing or modifying to support a new platform.
Before work on porting the FEA can begin, it will almost certainly be necessary to change the autoconf and automake files to support the new platform and to make compilation fixes for any differences for the new platform that are not already covered. It would be preferable for these changes be orthogonal to existing platforms so the changes can be integrated into the source tree with minimal work (assuming the porter wants to contribute the changes back.