First ensure Netpbm (https://netpbm.sourceforge.net/) is installed.
Then run:

	c99 PNNcolourmap.c -l netpbm -l m

Troubleshooting:
On my system, the package manager's netpbm was too old. (As indicated on
the Netpbm website, this affects certain Debian/Ubuntu versions.) After
downloading and installing from SourceForge, c99 would not recognize the
library because its filename did not follow the format lib*.so. To remedy
this I needed to make a link:

	ln -s /usr/lib/libnetpbm.so.11 /usr/lib/libnetpbm.so

