openssl: fix static build

If configured with no-dso, the openssl binary still gets linked with
-ldl, while libdl is not available for static-only builds. The OpenSSL
framework does not support automatically dropping -ldl with no-dso, so
-ldl has to be removed from Makefile after calling ./Configure as
explained in PROBLEMS.

Fixes http://autobuild.buildroot.org/results/cbb35f2582f1cfeb3d11cb4952a80784ff660baf



[Peter: add reference to PROBLEMS file]
Signed-off-by: Benoît Thébaudeau <benoit@wsystem.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>