bind: fix static openssl linking

Apparently LIBS (environment) isn't pushed to LIBS (autoconf) for the
tests, hence when static linking openssl it misses libz and assumes some
basic digests required for ecdsa support are missing, which is false. Fixes:
http://autobuild.buildroot.net/results/204/20411901d1bc6811e1ef8fa39457257d6bf1a146/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>