BUG: incorrect CMake variable for system jpeg include

This was encountered when doing some weird cross-compilation.

FindJPEG defines the following:
  JPEG_INCLUDE_DIR, where to find jpeglib.h, etc.
  JPEG_LIBRARIES, the libraries needed to use JPEG.
  JPEG_FOUND, If false, do not try to use JPEG.

Change-Id: I565b9c8b1399a1e2973ec5b4813497b1df64add0