Stop linking with -lnsl on Linux

[Martin Buchholz]

Hi Kelly, I'd like you to do a code review.

The JDK links with libnsl on Linux, but this is an unused dependency.

Here's a patch to stop linking with it - very similar to the handling
of libsocket:

http://cr.openjdk.java.net/~martin/webrevs/openjdk7/libnsl

I don't know whether libnsl is also unused on Solaris - I'll leave
that up to someone else.  Solaris and Linux libnsls are very
different.

Thanks,

Martin