package/nettle: fix shared-only build

Changeset f1d3e098 (Build shared libraries only as the default) broke the
nettle package.

nettle really wants to statically link its tools, examples and testsuite
against its own libraries. But for a shared-only build, static libraries
are not even built, so the build breaks when trying to link any of them.

Fix that in two ways:

  - the proper way, by using whatever libraries are available, with a
    preference toward using shared libraries;

  - disabling both examples and testsuite since we do not need them.

Fixes numerous build failures:
    http://autobuild.buildroot.net/results/e93/e93da743c5b84b1bb6274aeee3fe3b52962c560c/
    http://autobuild.buildroot.net/results/51d/51d0138ff39c45603315dcedf1f654c18604e1a8/
    http://autobuild.buildroot.net/results/521/521d979658e402103f1a18fcfeab3f9705f0c000/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>