bug#61988: error compiling emacs

[Eli Zaretskii <eliz@gnu.org>]

> From: Po Lu <luangruo@yahoo.com>
> Cc: utomei@tiscali.it,  61988@debbugs.gnu.org
> Date: Tue, 07 Mar 2023 08:34:18 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > I'd like to know why requiring libwebpdecoder is essential for the
> > build to work on the OP's system.  It isn't on mine, and if you look
> > at init_webp_functions, you will see that on Windows we load functions
> > only from libwebp and libwebpdemux.  Why is the third library needed?
> 
> It is here, because Emacs needs the function ``WebPGetInfo'', and on
> GNU/Linux systems that function is in libwebpdecoder.so:

Then the unconditional test for libwebpdecoder being installed is not
TRT.  We should instead do that only if that library is required, for
example by trying to link a program that calls WebPGetInfo without
libwebpdecoder first.

> and here, I have a pkg-config file named libwebpdecoder.pc

I see that file in the source directory, but it is not installed by
"make install", and neither is libwebpdecoder library itself.
Evidently, the way I configured and built libwebp doesn't require that
library to be installed.  And once again, in my case the offending
function is in libwebpdemux, not in libwebpdecoder.

So the test needs to be rewritten to be safe and correct.

> GNU ld does not allow you to link to symbols in dependencies of shared
> libraries, so asking for libwebpdemux while using symbols in
> libwebpdecoder does not work.

I know, and that's not what I suggested.  It looks like at least on
some systems (or maybe with some versions of libwebp) the
libwebpdecoder library is not installed and not needed.  We must
support such installations; we did until now.