scrypt: Disable OpenCL by default

README.scrypt

@@ -8,8 +8,12 @@ Scrypt mining for GPU is completely different to sha256 used for bitcoin
 mining. It has very different requirements to bitcoin mining and is a
 lot more complicated to get working well. Note that it is a ram dependent
 workload, and requires you to have enough system ram as well as fast enough
-GPU ram. If you have less system ram than your GPU has, it may not be possible
-to mine at any reasonable rate.
+GPU ram.
+
+Since scrypt ASICs are widely available, GPUs are not used by default. If you
+want to use them, use the -S opencl:auto option. Note that scrypt ASICs are
+documented in README.ASIC rather than this file, which only deals with GPU
+mining.
 
 There are 5 main parameters to tuning scrypt, all of which are optional for
 further fine tuning. When you start scrypt mining with the --scrypt option,

malgo/scrypt.c

@@ -581,6 +581,7 @@ struct mining_algorithm malgo_scrypt = {
 	.hash_data_f = scrypt_hash_data,
 	
 #ifdef USE_OPENCL
+	.opencl_nodefault = true,
 	.opencl_oclthreads_to_intensity = opencl_oclthreads_to_intensity_scrypt,
 	.opencl_intensity_to_oclthreads = opencl_intensity_to_oclthreads_scrypt,
 	.opencl_min_oclthreads =      0x100,  // intensity   8