Added new C macro ELI_USE_QD to configuration that user sets if they want to use QD in their build. Now, QD info only gets used if both ELI_QD_FOUND and ELI_USE_QD are defined. If they are both defined, then the macro ELI_USING_QD is defined. That is what all source code should test for if adding QD specific code. This fixes #9.