libobs: Stop configure if ImageMagick is preferred, but not found

With certain combinations of CMake (<3.9) and ImageMagick (>=7),
configure will silently fail setting up support for ImageMagick,
even if the user requested for it, and the build will move on
(as long as FFMPEG_AVCODEC is found).  This commit adds a check
that will stop configure in such cases.

Closes jp9000/obs-studio#1018

libobs/CMakeLists.txt

@@ -32,6 +32,8 @@ find_package(ImageMagick QUIET COMPONENTS MagickCore)
 
 if(NOT ImageMagick_MagickCore_FOUND AND NOT FFMPEG_AVCODEC_FOUND)
 	message(FATAL_ERROR "Either MagickCore or Libavcodec is required, but both were not found")
+elseif(NOT ImageMagick_MagickCore_FOUND AND LIBOBS_PREFER_IMAGEMAGICK)
+	message(FATAL_ERROR "ImageMagick support was requested, but was not found.")
 endif()
 
 option(LIBOBS_PREFER_IMAGEMAGICK "Prefer ImageMagick over ffmpeg for image loading" OFF)