Implement wolfSSL support for libssh2 when building with CMake.
Configuration example from curl-for-win:
```
-DCRYPTO_BACKEND=wolfSSL
-DWOLFSSL_LIBRARY=/path-to/wolfssl/lib/libwolfssl.a
-DWOLFSSL_INCLUDE_DIR=/path-to/wolfssl/include
```
Module `cmake/Findwolfssl.cmake` copied from:
e4d920c4b7/cmake/Findwolfssl.cmake
via commit:
296396d373
by Stefan Eissing
