Avoid unnecessary library linkage

* dirmngr/Makefile.am: Avoid $(DNSLIBS) for dirmngr_ldap
* g10/Makefile.am: $(LIBREADLINE) is only for gpg2; gpgv2 does not
need $(LIBASSUAN_LIBS)
* sm/Makefile.am: gpgsm does not need $(ZLIBS)
* tools/Makefile.am: gpgconf does not need $(NPTH_LIBS)

In the course of building GnuPG 2.1.0 beta864 on debian, i found that
several of the installed executables were linked to libraries that
they did not need to be linked to, which would cause unnecessary
package dependencies at runtime.

The changeset here removes these unnecessary libraries from linking.

Something similar could possibly also be done by passing --as-needed
to the linker, but trimming the depenencies seems more parsimonious.