System-wide dynamic libraries usually (always?) have NEEDED and RUNPATH
tags in a dynamic section (as `readelf -d /usr/lib/lib<...>.so` shows),
so when we link, say, with libssl.so, which depends on libz.so, a linker
does not complain against unresolved symbols that can be found in Z
library (if it is installed within a system).

Things are different when we linking with a static library. Say, when we
linking with libssl.a, which contains an unresolved symbol from Z
library, a linker reports an error. It is not possible to store an
information where to find unresolved symbols (NEEDED / RUNPATH) in a
static library (AFAIK).

We depend on three libraries that are depend on Z library: libcurl,
libssl and libcrypto (two latter are part of OpenSSL). When one of those
libraries is linked statically we should link with libz.so or libz.a
(depending on BUILD_STATIC flag). The patch doing exactly this.

The patch changes OPENSSL_LIBRARIES variable to fix the issue with
static linking of OpenSSL libraries. It also changes CURL_LIBRARIES in
the same way, however this does not alter any visible behaviour, because
OPENSSL_LIBRARIES is added to CURL_LIBRARIES. The latter change was made
to unify the way to choose libraries to link with: it is pure
refactoring part.

Fixes #4437.

(cherry picked from commit 2cdfaf3b)