cmake: Backport to work with CMake 3.0 again

Changes in commit 7867aaa9 (cmake: link curl to the OpenSSL targets
instead of lib absolute paths, 2018-07-17) and commit f826b4ce (cmake:
bumped minimum version to 3.4, 2018-07-19) required CMake 3.4 to fix
issue #2746.  This broke support for users on older versions of CMake
even if they just want to build curl and do not care whether transitive
dependencies work.

Backport the logic to work with CMake 3.0 again by implementing the
fix only when the version of CMake is at least 3.4.