Include <netinet/in.h> and <sys/socket.h>

FreeBSD (10.0) requires <netinet/in.h> and <sys/socket.h>
to be included.
Switch compiler to clang, set CFLAGS and LDFLAGS

Workarounds to build on FreeBSD 10.0:
1. Switch compiler and linker from gcc to clang
2. Extent the CFLAGS with -I$(PREFIXDIR)/include
3. Extent the LDFLAGS with -L$(PREFIXDIR)/lib