When checking for <net/if.h>, #include <sys/socket.h> first.
On Mac OS X 10.5.4, <net/if.h> does not #include <sys/socket.h> but
uses struct sockaddr defined there. Autoconf 2.61 generates a
configure script that warns if the header can be preprocessed but not
compiled. The Autoconf manual cautions that future versions of
Autoconf will treat the file as missing in this case. To let ELinks
detect <net/if.h> even with a future Autoconf, make the test program
#include <sys/socket.h> before <net/if.h>.