Something like build-webkit --libdir=/path/to/foo is currently not possible. Options passed to build-webkit and not processed before./configure time should be passed as arguments to ./configure. I will attach a patch ;)
Created attachment 49220 [details] proposed patch
Ugh, sorry, I just reviewed bug #35454 which does exactly the same thing. So closing this as duplicate. *** This bug has been marked as a duplicate of bug 35454 ***