Things still to be done:

* Compile netpbm under MS-DOS.

* Merge pgmnorm and ppmnorm into pnmnorm.

* Add a pnmtoraw filter, producing raw bytes on the output.

* Add better support for jpeg.

* Try to remove all global variables from the separate filters.
  They are a nuisance when building a merged binary.

* Clean up the nasty code bits.

* Add an automatic configuration script.
