opencl: Add missing TIFF library for macOS

The OpenCL code of Tesseract uses TIFF functions, but the TIFF library
was not added to the linker flags for macOS.

This fixes builds with OpenCL on Mac.
Signed-off-by: NStefan Weil <sw@weilnetz.de>

configure.ac

@@ -226,7 +226,7 @@ case "${host_os}" in
       fi
       AM_CPPFLAGS="-DUSE_OPENCL $AM_CPPFLAGS"
       OPENCL_CPPFLAGS=""
-      OPENCL_LDFLAGS="-framework OpenCL"
+      OPENCL_LDFLAGS="-framework OpenCL -ltiff"
     fi
     ;;
   *)