savefontdlg.c: Remove dead code (pre-validation threading)
All the threading code here is unused.
Looking at the git history, it appears that it was part of some
pre-validation idea that got canned along the way.

This commit: 36244a703185d22c9466f73a824ca54a6529fd5e
Removed the (#if 0'ed) code without removing the pthread_*
code as well.
Come to a hard-stop if user wants libreadline, but it's not available.

Default is to check and include, or not include if not available, but
if user option is "--with-libreadline" then the library and header files
must be available, otherwise ./configure comes to a hard-stop.
Add support for giflib version >= 5.1.0

DGifCloseFile has changed.
NB: Old commit here: fb0c4e6a5eca7a395f1c71964f7223308d6699db
Merge pull request #1404 from jtanx/threading

Some code cleanup & add support for GIFLIB >= 5.1.0
Merge pull request #1406 from JoesCat/check_headers_and_libraries_readline

Come to a hard-stop if user wants libreadline, but it's not available.
Merge branch 'master' into ufo_libxml2_1
Fix an infinite loop that would never run anyway.