Buildbot: Attempt to work-around unsupported clang on OSX machine

Ideally this would need to be fixed by nvcc itself. but they didn't
do that for a long time.

build_files/buildbot/slave_compile.py

@@ -74,6 +74,7 @@ if 'cmake' in builder:
         # Set up OSX architecture
         if builder.endswith('x86_64_10_6_cmake'):
             cmake_extra_options.append('-DCMAKE_OSX_ARCHITECTURES:STRING=x86_64')
+        cmake_extra_options.append('-DCUDA_NVCC_EXECUTABLE=/usr/local/cuda-hack/bin/nvcc')
 
     elif builder.startswith('win'):
         install_dir = None