Also allow to use #undef in OpenCL C code

Author: ProjectPhysX

src/kernel.hpp

@@ -10,6 +10,7 @@ string get_opencl_c_code() {
 	r = replace(r, "#ifdef\n", "#ifdef "); // except for the arguments after some preprocessor options that need to be in the same line
 	r = replace(r, "#ifndef\n", "#ifndef ");
 	r = replace(r, "#define\n", "#define "); // #define with two arguments will not work
+	r = replace(r, "#undef\n", "#undef ");
 	r = replace(r, "#if\n", "#if "); // don't leave any spaces in arguments
 	r = replace(r, "#elif\n", "#elif "); // don't leave any spaces in arguments
 	r = replace(r, "#pragma\n", "#pragma ");