Fixed GPU compile flag

src/openalpr/detection/detectorfactory.cpp

@@ -11,12 +11,13 @@ namespace alpr
     }
     else if (config->gpu_mode == 1)
     {
-      #if COMPILE_GPU
+      #ifdef COMPILE_GPU
       return new DetectorCUDA(config);
       #else
       std::cerr << "Error: GPU detector requested, but GPU extensions are not compiled.  " <<
               "Add COMPILE_GPU=1 to the compiler definitions to enable GPU compilation." <<
               std::endl;
+	  return new DetectorCPU(config);
       #endif
     }
   }