This allows feh to load .gif images via libcurl, as some imlib2 versions only
load gif images if the suffix is correct. It's also more convenient when using
--keep-http.

To achieve this, feh needs to use mkstemps. mkstemps is a non-standard
extension that is available on several systems. Compile feh with
"mkstemps=0" to use mkstemp instead.

Closes #630