Pin SVN dependencies

[paul-marechal]

One issue right now with the build system is that SVN dependencies always are pulled at their latest revision. This is specifically bad when we want to build oldest commits, since dependencies can be incompatible with how the code expects them to be when it was first written.

Without going too far back into the commit history, even in the situation where master isn't updated for some time, maybe something pulled from SVN will be different and break the build.

One approach would be to commit a file containing the current SVN revision hash (or similar) and to use that in the build system when pulling any of the dependencies. A bit like a lock file.

[paul-marechal]

cc @BluePrintRandom