Split libappgegl into libappgegl-generic and libappgegl-sse2, and
move the SSE2 code (part of the newly added smudge code) to the
latter, so that the rest of the code can be compiled without SSE2
compiler flags. This allows building GIMP with SSE acceleration
enabled, while running the resulting binary on a target with no
SSE accelration.
Ell