cmGlobalGenerator: Add ComputeHomeRelativeOutputPath method.
Fix generation of tgt/fast build targets. Commit 363caa2f (cmLocalGenerator: De-virtualize Configure()., 2015-05-30) moved the computation of HomeRelativeOutputPath from Configure-time to Generate-time, because it is only used at Generate-time. However, that commit caused the member for one local generator to be computed immediately before generating with that local generator, whereas previously the members of all local generators were computed before generating any of them. The HomeRelativeOutputPath is used by the GetRelativeTargetDirectory method, which is called by the cmGlobalUnixMakefileGenerator3::WriteConvenienceRules method. That method is called by the cmLocalUnixMakefileGenerator3::WriteLocalMakefile method when generating for the top-most (ie, the first) local generator. At that point, the HomeRelativeOutputPath is not yet computed. Fix that by computing the member just before generating anything. This will eventually be done in the cmLocalUnixMakefileGenerator3 constructor instead, but further refactoring is needed to make that possible.