libusb1: avoid -lgcc_s on darwin

[kowey]

I confess I don't really understand the implications of this, but other packages seem to use the same fix, stating only that libgcc_s isn't available. I'm just trying to make gnupg build! (it depends on libusb)

[vcunat]

Hmm, I didn't know that our gccApple doesn't have/need libgcc_s.

[Phreedom]

it looks like we need a more generic way to handle this. maybe add a gcc
property that evaluates to proper ld flags to link/not link against this lib?

[vcunat]

It would be best if we could patch gcc at configure-time to always link those libs iff they are needed.