Compiling autotrace against GraphicsMagick instead of ImageMagick

In an extravagant instance of yak-shaving, I found myself trying to compile autotrace so that I could use it to trace my handwriting samples so that I could make a font with FontForge so that I could make worksheets for A- without worrying about finding a handwriting font with the right features and open font licensing.

AutoTrace had been written against ImageMagick 5, but this was no longer easily available. ImageMagick 7 greatly changed the API. Fortunately, GraphicsMagick kept the ImageMagick 5 API. I eventually figured out enough about autoconf and C (neither of which I had really worked with much before) to switch the library paths out while attempting to preserve backward compatibility. I think I got it, but I can’t really tell aside from the fact that compiling it with GraphicsMagick makes the included tests run. Yay!

At first I tried AC_DEFINE-ing twice in the same package check, but it turns out only the last one sticks, so I moved the other AC_DEFINE to a different condition.

Anyway, here’s the pull request. Whee! I’m coding!