bug#20702: 25.0.50; (elisp) `ImageMagick Images': say how to tell if Emacs has IM support, etc.

[Drew Adams <drew.adams@oracle.com>]

Someone using the Elisp manual might not have built the Emacs build
being used.  S?he might not even know what build a user might use.

This node should tell an Emacs-Lisp programmer how, using Lisp, to tell
whether the current build has ImageMagick support.

IOW, the opening phrase of this node, "If you build Emacs with
ImageMagick support" should be replaced by "If an Emacs build has
ImageMagick support", and this intro should be followed by telling users
how to check this using Lisp.

My guess (from trying) would be that (fboundp 'imagemagick-types) is the
test that tells you this - in my build it returns nil.

In any case, the doc should also say explicitly that this function is
not defined if the Emacs build has no ImageMagick support.  In
particular, the way `imagemagick-types' is described now, one could
reasonably (and mistakenly) expect that in an Emacs build that has no
ImageMagick support `imagemagick-types' would be defined but would
return () - no ImageMagick image types are supported.  (And why isn't
this the case?)

In sum, the intro to this node needs to be written for Elisp programmers
in general.


In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'