Some of the generic files still contain SSE2 code, in particular
gimpoperationlayermode.c. The reason why it often works without is that
native gcc will usually pre-define SSE macros by default.
To check this: gcc -dM -E - < /dev/null | grep SSE
Yet I had a case on a small netbook where the SSE macros were not
pre-defined even though supported. Consequently the build failed.