[ gpsdrive ] building with GDAL support

[Hamish]

I've commented out the calls to drawmarkers() in the GDAL code so that it
builds now. No idea what happened to the code for that function, it
appears that it is a local fn which should be in lib_map/ but isn't.

I'm not sure what the two util programs, worldgen and gmapview do.
AFIACG worldgen creates a "world file" (see Docs/README.lib_map), and
gmapview is a tile viewer of some sort. I could get it to run with 
"gmapview -h", but didn't get very far. Maybe it needs a world file
for the tile first?

shrug,
Hamish