Libccv is an open source image classifier and computer vision library.
It runs on Mac OSX, Linux, FreeBSD, Windows, iPhone, iPad, Android, Raspberry Pi. In fact, anything that has a proper C compiler probably can run ccv. The majority (with notable exception of convolutional networks, which requires a BLAS library) of ccv will just work with no compilation flags or dependencies.
Among lots of other things it has a SIFT implementation
The sources are available on github.