Need to rebuild targets when their config changed

[GPMueller]

The question is whether all targets from a project file should be rebuilt when the file is changed, or whether we should actually track changes per target.
The latter could be done with a meta-information file inside the target build folder.

[GPMueller]

It seems to me the way to implement this would not be to compare the modification time of a project's config file against the dates of object files etc.
Instead, complimentary to issue #48, we could use hashlib to hash the config of each target and save it as build/.../targetname/buildtype/hash/config.hash. On building a target, we can check whether the config hash has changed and, if so, rebuild the whole target.