Default to using gmake if MAKE is not already set.
Default to using gmake if MAKE is not already set.

-C is a GNU make feature, and libusdt requires GNU make, so we default
to gmake here.

mgenereu/node-dtrace-provider,DataSweeper/node-dtrace-provider,tjfontaine/node-dtrace-provider,tjfontaine/node-dtrace-provider,DataSweeper/node-dtrace-provider,dieface/node-dtrace-provider,mgenereu/node-dtrace-provider,DataSweeper/node-dtrace-provider,dieface/node-dtrace-provider,dieface/node-dtrace-provider,mgenereu/node-dtrace-provider,dieface/node-dtrace-provider,DataSweeper/node-dtrace-provider,tjfontaine/node-dtrace-provider,tjfontaine/node-dtrace-provider,mgenereu/node-dtrace-provider