build with nat-pmp from pkgconfig if available

Change-Id: Idc5ae4611a99132336ff62617d66b842fc87323a

Author: aberaud

CMakeLists.txt

@@ -218,6 +218,10 @@ if (DHTNET_NATPMP AND NOT MSVC)
             message("NAT-PMP found: ${natpmp_LIBRARIES}")
             set (libsprivate "${libsprivate} -lnatpmp")
         endif()
+    else()
+        message("NAT-PMP found")
+        set(natpmp_LIBRARIES PkgConfig::natpmp)
+        set (requiresprivate "${requiresprivate} libnatpmp")
     endif()
 endif()