GDAL dependency info and /bin/bash default for make

I just generated the swiss map successfully but it was a somewhat long way to get here.

You highly depend on the GDAL library but don't mention it in the readme. A short note will save a lot of time for others. 

On my ubuntu machine make automatically falls back to /bin/sh for shell scripts causing the error "sh 7: i: not found" on line 195 of the Makefile which repeats endlessly. Using /bin/bash instead of /bin/sh solves the problem.
Make prefers the SHELL variable if set.

Thanks for this great project!