shotdetect on os x

lemac:Shotdetect xx$ mkdir -p build; cd build; cmake -D USE_WXWIDGETS:BOOL=OFF ..
-- The C compiler identification is Clang 5.0.0
-- The CXX compiler identification is Clang 5.0.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- avformat library found: /usr/local/lib/libavformat.dylib
-- avdevice library found: /usr/local/lib/libavdevice.dylib
-- avcodec library found: /usr/local/lib/libavcodec.dylib
-- avutil library found: /usr/local/lib/libavutil.dylib
-- swscale library found: /usr/local/lib/libswscale.dylib
Found all FFmpeg libraries in /usr/local/lib/libavformat.dylib;/usr/local/lib/libavdevice.dylib;/usr/local/lib/libavcodec.dylib;/usr/local/lib/libavutil.dylib;/usr/local/lib/libswscale.dylib.
CMake Error at cmake/FindGD.cmake:112 (MESSAGE):
  Could not find GD library
Call Stack (most recent call first):
  CMakeLists.txt:85 (FIND_PACKAGE)