Added build_libjpeg_turbo

[radarhere]

This PR suggests adding build_libjpeg_turbo.

This is an alternative to libjpeg, so rather than touching "jpegturbo-stamp" or similar, it touches "jpeg-stamp", just as build_jpeg does. This means that if a user ran build_libjpeg_turbo, and then build_tiff (which internally calls build_jpeg), libjpeg would not be built, because libjpeg turbo was already in place.

[mattip]

Could you add this to the tests?

[radarhere]

Ok, done. In the tests, I've first removed jpeg-stamp to allow it to be tested in addition to build_jpeg.

[mattip]

Thanks