build: use define flags for proxy

CMakeLists.txt

configure.ac

include/opendht.h

include/opendht/dht_proxy_client.h

include/opendht/dht_proxy_server.h

src/dht_proxy_client.cpp

src/dht_proxy_server.cpp

src/dhtrunner.cpp

src/securedht.cpp

tools/dhtchat.cpp

tools/dhtnode.cpp