bug#8640: 24.0.50; no pbm support on cygwin

[Katsumi Yamaoka]

Hi,

pbm image is not available in Emacs trunk built on cygwin at least
after May 6: (image-type-available-p 'pbm) => nil
Whereas png, jpeg, gif, xpm, etc. are all ok.  I guess this is due
to the recent changes in image.c.  Any hint?  Thanks.

In GNU Emacs 24.0.50.1 (i686-pc-cygwin, X toolkit, Xaw3d scroll bars)
 of 2011-05-09 on localhost
Windowing system distributor `The Cygwin/X Project', version 11.0.11000000
configured using `configure  '--verbose' '--with-x-toolkit=lucid''
[...]