package/giflib: bump to version 5.2.1

package/giflib: bump to version 5.2.1

- Switch to generic-package (autotools has been dropped since version
  5.1.5)
- Remove hook and instead use dedicated makefile targets to build only
  shared or static library and not binaries or documentation (added by
  an upstreamble patch)
- ac_cv_prog_have_xmlto=no can be removed as doc is not built anymore

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Peter: drop redundant GIFLIB_SOURCE]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>