Problem: libpng (a prereq of ImageMagick) has trouble building on OS X x86.
Fix: The fix is two-fold. First you must upgrade libpng from 1.2.10 to 1.2.16. Then you must apply the attached patches to it. These two steps fix the problems. (Note that one of the patches is actually for the ImageMagick code that interfaces with libpng because the libpng patch requires a change there too.)