Custom build path pulls git gems for every build target

Introduced in #1384, a git based gem will be cloned to the current build target's build directory. 

This works fine but the gem is cloned for every build target, adding a massive overhead when running clean/make regularly.

Is it acceptable to add ```gem_clone_dir``` as an option to ```build_config.rb```, or is it an unnecessary overhead to programmers with a custom build path? (@xxuejie)