Makefile: Avoid double path separator (//)
Makefile: Avoid double path separator (//)

Causes problems on Windows/Cygwin. Fixes #471.

twahlfeld/pianobar,bmbove/pianobar-save,gnowxilef/pianobar,johnso51/pianobar,tchajed/pianobar,johndeeney/pianobar,johnso51/pianobar,johndeeney/pianobar,bmbove/pianobar-save,gnowxilef/pianobar,johndeeney/pianobar,bmbove/pianobar-save,johnso51/pianobar,gnowxilef/pianobar,tchajed/pianobar,twahlfeld/pianobar,gnowxilef/pianobar,johndeeney/pianobar,DavidZemon/pianobar,DavidZemon/pianobar,tchajed/pianobar,tchajed/pianobar,DavidZemon/pianobar,twahlfeld/pianobar,DavidZemon/pianobar,tchajed/pianobar,johnso51/pianobar,johnso51/pianobar,johndeeney/pianobar,bmbove/pianobar-save,gnowxilef/pianobar,johnso51/pianobar,twahlfeld/pianobar,twahlfeld/pianobar,DavidZemon/pianobar,bmbove/pianobar-save,twahlfeld/pianobar,tchajed/pianobar,gnowxilef/pianobar,DavidZemon/pianobar,johndeeney/pianobar