new image library "requirements"

[Miles Bader <miles@gnu.org>]

I just tried compiling a new emacs, and the configure failed with:

   configure: error: The following required libraries was not found:
        libgif/libungif libtiff
   Maybe some development libraries/packages are missing?
   If you don't want to link with them give
        --with-gif=no --with-tiff=no
   as options to configure

While there's a case to be made for "requiring" (without options) X
libraries or even jpeg /png, I think requiring these is kind of silly.

Tiff is fairly rarely used except in specialized applications.

The case with .gif is more vague, but it seems largely a legacy format,
and is getting more and more rare these days -- you see it on old web
sites, and people still use it for "animated" images (which emacs
doesn't support), but it seems kind of unusual to see it for the kind of
content which people might keep around.

At the least, I'd drop the "requirement" for tiff.

-miles
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