opencv3: add enableIpp flag

Intel Integrated Performance Primitives (IPP) speeds up parts of OpenCV
on Intel processors (and compatible). It increases the store path from
220 MiB to 300 MiB, so it defaults to off.

Original patch from Bas van Dijk <v.dijk.bas@gmail.com>.

I tried applying the same change to opencv(2.x). OpenCV 2.x didn't
automatically detect IPP, so I reverted the change.