configure: Moved detection of GIF support to a dedicated macro

configure: Moved detection of GIF support to a dedicated macro

The original check was not compliant with autoconf's syntax, did not
have a very good behaviour for user and was not easy to make evolve.

The new macro:
 - uses as much as possible autoconf macros for portability and code
consistency;
 - provides a consistent behaviour on yes/no/auto (if user explicitly
enables support, do not silently disable if not found; if library is found
but not the header, complain to let user install it or explicitly disable
support);
 - makes uses of shell functions to keep generated configure smaller by
sharing reusable stuff;
 - introduces a tracking of unsupported things to be reported to user

It includes a fixes from a problem found by Amadeusz S\e-B³awiñski, because\e-A
the function 'wm_fn_imgfmt_try_link' modified the variable LDFLAGS
instead of LIBS.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>