Add variables for pamscale/pnmscale

I'm using ubuntu 12.04 and mew 6.5.
I couldn't see jpeg iamge of attached file.
Because my environment didn't have pamscale.
(I updated netpbm and installed netpbm-dev too)
But pnmscale was ok.
So I attached variables for pamscale/pnmscale to change the program and option.
Could you merge this commit?