configure.in: only add -lnsl when needed

configure.in: only add -lnsl when needed

dpkg-shlibdeps: warning: dependency on libnsl.so.1 [..]
(they use none of its symbols).

As shown via commit 2b14234abc252383679bae2d23861b773dc9713e
(dpkg-shlibdeps: warning: dependency on libnsl.so.1) and the
following revert of that commit, we cannot simply drop
the AC_CHECK_LIB(nsl). Although -lnsl is indeed unneeded
when glibc is used, some platforms (e.g. Solaris) need it.

Use AC_SEARCH_LIBS instead to only link when the library exports
a particular symbol.