Turning off compilation for GPU detector by default.

Compiler flag can be used to re-enable it

src/openalpr/detection/detectorfactory.cpp

@@ -11,7 +11,13 @@ namespace alpr
     }
     else if (config->gpu_mode == 1)
     {
+      #if 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;
+      #endif
     }
   }