Use poppler_document_new_from_gfile() instead.

Apparently file:// URIs are not portable.

Remove the build time check for poppler_document_new_from_fd()

Use poppler_document_new_from_file()

Initially motivated by finding that the version of Poppler
packaged for Cygwin doesn't yet have poppler_document_new_from_fd(),
I've come to discover that this is actually slightly cleaner.

Initial commit