Alist of dynamic libraries vs external files implementing them.
Each element is a list (LIBRARY FILE...), where the car is a symbol
representing a supported external library, and the rest are strings giving
alternate filenames for that library.
Emacs tries to load the library from the files in the order they appear on
the list; if none is loaded, the running session of Emacs won't have access
to that library.
Note that image types `pbm' and `xbm' do not need entries in this variable
because they do not depend on external libraries and are always available.
Also note that this is not a generic facility for accessing external
libraries; only those already known by Emacs will be loaded.