Add ImageMagick support

[yoursdearboy]

This PR adds ImageMagick support and fixes #1.

Build script installs ImageMagick 6 dependency via brew and links it manually (because imagemagick@6 is a keg-only formula and we don't need another imagemagick formula anyway).

Then copies shared libraries as usual and coders directory manually to the bundle.

To make lookup work I added three new environment variables to startup script (see this SO question). But I'm not sure whether it's a good idea to set DYLD_LIBRARY_PATH.

Tested with Mac OS 10.14.6.

[dsedivec]

Just a note that Emacs HEAD, "no longer defaults to using ImageMagick to display images,
due to security and stability concerns", and will no longer build with ImageMagick support by default AFAIK.

[yoursdearboy]

Does it mean that by default (without the flag) we won’t be able to manipulate images (resize, rotate, etc)? I use it to display images miniatures. I think this is an important feature for org-mode users to enable by default.

[dsedivec]

Does it mean that by default (without the flag) we won’t be able to manipulate images (resize, rotate, etc)? I use it to display images miniatures. I think this is an important feature for org-mode users to enable by default.

Sound like no, you won't need ImageMagick, at least not to resize and rotate. I found this in master's etc/NEWS:

Emacs now supports resizing and rotating images without ImageMagick. All modern systems support this feature. (On GNU and Unix systems, Cairo drawing or the XRender extension to X11 is required for this to be available; the configure script will test for it and, if found, enable scaling.)