Make SHA256d mining optional
Bugfix: cointerra: Can handle nonces down to pdiff 1
Significantly rewrite configure script to handle driver dependencies cleaner
opencl: Only attempt to BFI_INT patch SHA256d kernels
cpu: Remove redundant checks for USE_CPUMINING
opencl: Remove redundant checks for USE_OPENCL
Merge branch 'opt_sha256d' into bfgminer