bugfix for missing mechanism paths

The mechansims/gpu and mechanisms/multicore paths were not being generated
by CMake during configuration, and modcc was silently ignoring that it was
not able to write to the nonexistant target paths.
I have updated CMake to generate the missing gpu and multicore target paths.

mechanisms/CMakeLists.txt

@@ -9,6 +9,12 @@ if(USE_OPTIMIZED_KERNELS) # generate optimized kernels
     set(modcc_flags ${modcc_flags} -O)
 endif()
 
+# make path for the kernels that will be generated by modcc
+file(MAKE_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/multicore)
+if(WITH_CUDA)
+    file(MAKE_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/gpu)
+endif()
+
 # generate source for each mechanism
 foreach(mech ${mechanisms})
     set(mod "${CMAKE_CURRENT_SOURCE_DIR}/mod/${mech}.mod")