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.