configure: Make WebP support togglable, and improve its error message.

configure: Make WebP support togglable, and improve its error message.

The current way was enabling WebP support whenever libwebp was found,
giving no way to the user to disable it if they had the library
installed but didn’t want to link against it.  This adds a
--without-webp configure option to never link against it, and a
--with-webp one to fail the build if it isn’t found, the default being
to use it if it is present.

Additionally, we now tell the user when WebP support has been disabled
and they try to load a WebP file.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>