Make presence of netinet/ip.h optional

Some environments, for example lwip, don't have it, so let's make
it configurable so we can also build and work on those platforms
by simplying using project configuration.

Author: fedepell

configure.ac

@@ -85,6 +85,7 @@ AC_CHECK_HEADERS([ \
     linux/serial.h \
     netdb.h \
     netinet/in.h \
+    netinet/ip.h \
     netinet/tcp.h \
     sys/ioctl.h \
     sys/params.h \

src/modbus-tcp.c

@@ -41,7 +41,9 @@
 #endif
 
 # include <netinet/in.h>
+#ifdef HAVE_NETINET_IP_H
 # include <netinet/ip.h>
+#endif
 # include <netinet/tcp.h>
 # include <arpa/inet.h>
 # include <netdb.h>