Use prebuilt ImageMagick when it's available.
Use prebuilt ImageMagick when it's available.

DYLD_LIBRARY_PATH needs to be set to allow ImageMagick tools to find the
shared library on OS X when the application isn't installed on the machine
(e.g it's running from our prebuilts directory).

Tested:
Verified it's possible to build Zooshi assets with ImageMagick
from prebuilts on Linux and OS X.

Bug: 23314076

Change-Id: I4b04592f8c3a4fe0e2c23d70d804914b2a4ab08f

google/zooshi,google/zooshi,google/zooshi,google/zooshi,google/zooshi