fix opencl kernel build on macosx

On OSX (10.9), some OpenCL kernels fail to build because Apple's compiler doesn't accept more than one space character between the arguments:

~~~
src $ CL_LOG_ERRORS=stdout ../run/john --te --fo=rar-opencl
OpenCL platform 0: Apple, 2 device(s).
Device 1: HD Graphics 5000
[CL_INVALID_BUILD_OPTIONS] : OpenCL Error : clBuildProgram failed: Invalid build options "-I ../run/kernels  -DDEVICE_INFO=258 -DAPPLE  -DHASH_LOOPS=8 -DPLAINTEXT_LENGTH=22"
Break on OpenCLErrorBreak to debug.
Build log: 
Error -43 building kernel. DEVICE_INFO=258
OpenCL error (CL_INVALID_BUILD_OPTIONS) in file (common-opencl.c) at line (222) - (clBuildProgram failed.)
src $
~~~

I saw that bleeding-jumbo branch fixes this issue, but unfortunately I can't compile it due to an unresolved MD5_body reference in the final linking stage (should I open a new issue about it?). So I propose this temporary fix while the other changes from the bleeding branch aren't merged.