FindOpenCL: add AMDAPPSDKROOT into paths for OpenCL library

This path was only looked up for windows.  The AMD SDK also exists for
Linux, and we may want to use the OpenCL library for there.

Modules/FindOpenCL.cmake

@@ -117,7 +117,11 @@ if(WIN32)
   endif()
 else()
   find_library(OpenCL_LIBRARY
-    NAMES OpenCL)
+    NAMES OpenCL
+      ENV AMDAPPSDKROOT
+    PATH_SUFFIXES
+      lib/x86_64
+      lib/x64)
 endif()
 
 set(OpenCL_LIBRARIES ${OpenCL_LIBRARY})