add flag for auto SHP creation

[nvkelso]

GeoJSON is nice, but SHP is more portable.

If ogr2ogr is present:

Note: ogr2ogr won't replace an existing file, so remove them, and in the clean make sure that dir exists

rm -f shp/z2-labels.* shp/z2-points.*
ogr2ogr -f "ESRI Shapefile" shp/z2-labels.shp geojson/z2-labels.json
ogr2ogr -f "ESRI Shapefile" shp/z2-points.shp geojson/z2-points.json

In the general Makefile,

clean:
mkdir -p shp

[nvkelso]

with the -overwrite flag on.