Change two Makefile.common var references to use $() over ${}.
Change two Makefile.common var references to use $() over ${}.

Both are legal in Makefiles, but $() matches local convention, and $()
prevents confusion about when the variable is expanded (i.e., shell vs.
make).

zklinger/vimperator-labs,nullren/vimperator-labs,derenio/vimperator-labs,5digits/dactyl,moonshadow/dactyl,Evidlo/vimperator-labs,airodactyl/dactyl,Kjwon15/dactyl,zklinger/vimperator-labs,airodactyl/airodactyl,invl/dactyl,airodactyl/dactyl,nathyong/dactyl,Quicksaver/vimperator-labs,airodactyl/airodactyl,madand/pentadactyl-pm,seanmorton/vimperator-labs,invl/dactyl,paretje/vimperator-labs,nullren/vimperator-labs,stemar94/vimperator-labs,airodactyl/dactyl,Kjwon15/dactyl,nathyong/dactyl,invl/dactyl,moonshadow/dactyl,moonshadow/dactyl,derenio/vimperator-labs,5digits/dactyl,moonshadow/dactyl,madand/pentadactyl-pm,mcarton/vimperator-labs,Kjwon15/dactyl,madand/pentadactyl-pm,Kjwon15/dactyl,Kjwon15/dactyl,Kjwon15/dactyl,Evidlo/vimperator-labs,Quicksaver/vimperator-labs,nathyong/dactyl,madand/pentadactyl-pm,paretje/vimperator-labs,Sheile/vimperator-labs,5digits/dactyl,nathyong/dactyl,madand/pentadactyl-pm,seanmorton/vimperator-labs,5digits/dactyl,airodactyl/dactyl,airodactyl/dactyl,invl/dactyl,moonshadow/dactyl,invl/dactyl,Sheile/vimperator-labs,stemar94/vimperator-labs,5digits/dactyl,nathyong/dactyl,nathyong/dactyl,airodactyl/dactyl,invl/dactyl,madand/pentadactyl-pm,moonshadow/dactyl,5digits/dactyl,mcarton/vimperator-labs