allow cmake to use imported mbedtls library

CMakeLists.txt

@@ -371,7 +371,9 @@ if (USE_GNUTLS)
 		target_link_libraries(datachannel-static PRIVATE Nettle::Nettle)
 	endif()
 elseif(USE_MBEDTLS)
-	find_package(MbedTLS 3 REQUIRED)
+	if(NOT TARGET MbedTLS::MbedTLS)
+		find_package(MbedTLS 3 REQUIRED)
+	endif()
 	target_compile_definitions(datachannel PRIVATE USE_MBEDTLS=1)
 	target_compile_definitions(datachannel-static PRIVATE USE_MBEDTLS=1)
 	target_link_libraries(datachannel PRIVATE MbedTLS::MbedTLS)