Xcode will autocreate scheme files once you open the project in xocde.
But if you do not, there is no way to build only part of a workspace if you don't specify a scheme.
A solution would be this mr:
premake/premake-xcode#25
@ratzlaff , @starkos was there a reason for which this was not merged?
Should I make a new MR with this? Ideally we should specify trough a command line option or a flag if we want schemes to be generated or not.