[openssl] added support for platforms without libdl

[qis]

Since some platforms lack a dl library, the vcpkg-cmake-wrapper.cmake script from the OpenSSL port should not indiscriminately add it to the link libraries list.

Tested on the following platforms with a project that uses LibSSH2:

• Linux (WSL on Windows 10 1709)
• FreeBSD (12.0-CURRENT r324496 amd64)

These changes have no effect on Windows builds.

P.S.: @ras0219-msft If you already have something better, just close this without commenting. :)

[ras0219-msft]

Thanks for making the PR, @qis!