configure.ac: fix ssl static build

[ffontaine]

Retrieve openssl dependencies through pkg-config to fix the following static build failure:

checking for SSL_ctrl  in -lssl... no
configure: error: libssl not found; install OpenSSL

Fixes:

• http://autobuild.buildroot.org/results/d3bbbdb8433f1da69304a42bc3f3d4a49ce52931

Signed-off-by: Fabrice Fontaine fontaine.fabrice@gmail.com