Enable HAS_SET_DIRECTION_ENABLED for regular build

It looks like the option to enable pcap direction doesn't get compiled
in during a manual build (unless I have missed something). The option
is a gmake variable and is indeed promoted in Tests/Pcap++Test/Makefile,
but never seemed to get promoted to a compiler macro argument in the
standard Makefile. This change should correct this.

Pcap++/Makefile

@@ -71,6 +71,10 @@ ifdef HAS_PCAP_IMMEDIATE_MODE
 DEPS += -DHAS_PCAP_IMMEDIATE_MODE
 endif
 
+ifdef HAS_SET_DIRECTION_ENABLED
+PCAPPP_BUILD_FLAGS += -DHAS_SET_DIRECTION_ENABLED
+endif
+
 ifdef USE_DPDK
 PCAPPP_BUILD_FLAGS += -msse -msse2 -msse3 -mssse3
 endif