fix: add missing bundle destination for Mac OS X

Set the BUNDLE DESTINATION to cwd.

The compiled application bundle (.app) will be installed here after it's compiled.

When unset, builds would fail with this error:

    CMake Error at CMakeLists.txt:209 (install):
      install TARGETS given no BUNDLE DESTINATION for MACOSX_BUNDLE executable
      target "shotdetect-gui".

Build environment:

    CMake: 2.8.11.2
    OS X: 10.8.4-x86_64