Fix EVP_PKEY_CTX_... compilation with LibreSSL

Fix EVP_PKEY_CTX_... compilation with LibreSSL

Commit 06f6cf3ff850f29 introduced use of newer OpenSSL functions
for the TLS 1.0-1.1 PRF, to make OpenVPN work with FIPS-enabled OpenSSL.

LibreSSL masquerades as "very new OpenSSL" but does not have these
functions (or at least not on the OpenBSD system tested), so compilationg
breaks.

Add a "but not if LibreSSL" check to the OpenSSL version check, as we
do in other places.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20210308114405.19066-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21628.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>