Fix generation of .pc when compiling with CMake.
Fix generation of .pc when compiling with CMake.

caizw/ortp,jiangjianping/ortp,Distrotech/oRTP,videomedicine/oRTP,Distrotech/oRTP,carpikes/ortp,jiangjianping/ortp,VTCSecureLLC/ortp,Linphone-sync/oRTP,wugh7125/ortp,Distrotech/oRTP,VTCSecureLLC/ortp,wugh7125/ortp,carpikes/ortp,caizw/ortp,carpikes/ortp,caizw/ortp,videomedicine/oRTP,Linphone-sync/oRTP,Linphone-sync/oRTP,wugh7125/ortp,VTCSecureLLC/ortp,jiangjianping/ortp,videomedicine/oRTP