I'm building a project that uses source from a project that's been built from a project that uses code under MIT and GNU licences.
What steps do I need to take to make sure that the source has the correct licence files in the right places?
The project in question is a php plugin for another app, not an executable.
If you mix code it must comply with both licenses, which generally means they must be compatible. I believe MIT is pretty easy to merge with GPL as long as you obey the following:
The MIT License
Copyright (c)
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
Which seems to imply the only requirement is to place the license file with an "substantial" portions of code. It should NOT override the GPL though because the language of GPL is more strict and requires the MIT portions to become GPL (in your version). In other words your whole software must be GPL.