Make build_curl more robust: clean up previous build attempts.
Make build_curl more robust: clean up previous build attempts.

andyshinn/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,olegnev/dx-toolkit,olegnev/dx-toolkit,johnwallace123/dx-toolkit,andyshinn/dx-toolkit,andyshinn/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,olegnev/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,olegnev/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,andyshinn/dx-toolkit,johnwallace123/dx-toolkit,johnwallace123/dx-toolkit,johnwallace123/dx-toolkit,andyshinn/dx-toolkit,olegnev/dx-toolkit,olegnev/dx-toolkit,andyshinn/dx-toolkit,dnanexus/dx-toolkit,andyshinn/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,jhuttner/dx-toolkit,johnwallace123/dx-toolkit,olegnev/dx-toolkit,dnanexus/dx-toolkit,olegnev/dx-toolkit,jhuttner/dx-toolkit,andyshinn/dx-toolkit