configure: fix net/if.h detection on osx (10.7)

configure.ac

@@ -2255,7 +2255,6 @@ AC_CHECK_HEADERS([ \
 arpa/inet.h \
 langinfo.h \
 features.h \
-net/if.h \
 netinet/in.h \
 netinet/tcp.h \
 sys/prctl.h \
@@ -2269,6 +2268,21 @@ sys/un.h \
  #endif
 ])
 
+AC_CHECK_HEADERS([net/if.h], [], [],
+[#include <stdio.h>
+#if STDC_HEADERS
+# include <stdlib.h>
+# include <stddef.h>
+#else
+# if HAVE_STDLIB_H
+#  include <stdlib.h>
+# endif
+#endif
+#if HAVE_SYS_SOCKET_H
+# include <sys/socket.h>
+#endif
+])
+
 ### Checks for types
 
 ### Checks for structures