configury: fix <net/if.h> detection

only invoke AC_CHECK_HEADERS([net/if.h]) once and with the required prerequisites

Thanks Paul Hargrove for the report and diagnostic

configure.ac

@@ -584,7 +584,7 @@ AC_CHECK_HEADERS([alloca.h aio.h arpa/inet.h dirent.h \
     libutil.h memory.h netdb.h netinet/in.h netinet/tcp.h \
     poll.h pthread.h pty.h pwd.h sched.h \
     strings.h stropts.h linux/ethtool.h linux/sockios.h \
-    net/if.h sys/fcntl.h sys/ipc.h sys/shm.h \
+    sys/fcntl.h sys/ipc.h sys/shm.h \
     sys/ioctl.h sys/mman.h sys/param.h sys/queue.h \
     sys/resource.h sys/select.h sys/socket.h sys/sockio.h \
     sys/stat.h sys/statfs.h sys/statvfs.h sys/time.h sys/tree.h \