Cleanup netipx/ipx.h availability check

* configure.ac (AC_CHECK_HEADERS): Add netipx/ipx.h.
* net.c: Check for HAVE_NETIPX_IPX_H instead of __GLIBC__.
diff --git a/configure.ac b/configure.ac
index 56c9b30..9eccde9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -394,6 +394,7 @@
 	linux/utsname.h
 	mqueue.h
 	netinet/sctp.h
+	netipx/ipx.h
 	scsi/sg.h
 	stropts.h
 	sys/conf.h