libusb: only require libgcc_s when using gcc4.
libusb: only require libgcc_s when using gcc4.

* Gcc2 does not provide this library, this would make the libusb package
not installable.

moochris/haikuports,Begasus/haikuports,Begasus/haikuports,moochris/haikuports,haikuports/haikuports,Begasus/haikuports,moochris/haikuports,koletzky/haikuports,haikuports/haikuports,haikuports/haikuports,nielx/haikuports,nielx/haikuports,koletzky/haikuports,haikuports/haikuports,Begasus/haikuports,Begasus/haikuports,nielx/haikuports,koletzky/haikuports,miqlas/haikuports,miqlas/haikuports,haikuports/haikuports,miqlas/haikuports