[gentoo-dev] make boehm-gc USE flag global

There are 4 packages with the USE flag boehm-gc:

dev-embedded/sdcc
gnustep-base/libobjc2
media-gfx/asymptote
net-libs/onion

plus 3 packages with the USE flag gc having the same meaning:

sci-mathematics/flint
sys-apps/nix
www-client/elinks

I think it would be reasonable to rename the flag gc to boehm-gc in these 
3 packages, and make boehm-gc global. The description can be something 
like

Enable garbage collection using dev-libs/boehm-gc

Andrey