pass DISABLE_Gadgetron to SIRF

[paskino]

Partly addresses #750 by setting DISABLE_Gadgetron=(NOT ${BUILD_Gadgetron}) in SIRF's external build.

This is however, still not entirely correct as it does not address:

1. USE_SYSTEM_Gadgetron=ON
2. the case in which BUILD_Gadgetron=ON but the user wants (why would they?) to disable Gadgetron's support in SIRF.

[KrisThielemans]

I'm not really sure if NOT works that way. Does it?

In any case, I don't think this will cope with what we want on Windows. There BUILD_Gadgetron=OFF but (at least normally) SIRF's DISABLE_Gadgetron=OFF.

I see no other way than introducing SIRF_DISABLE_Gadgetron.

[paskino]

It does work, as I used it to build on my Windows machine lately. However, it doesn't fullfil the requirements and SIRF_DISABLE_Gadgetron is a simpler solution.