Move `libssh2_config.h` generation and setting `-DHAVE_CONFIG_H` to
the root `CMakeFile.txt`.
Also move symbol hiding setup there. It needs to be done before
generating the config file for `LIBSSH2_API` value to be set in it.
After this change the `HIDE_SYMBOLS` setting is accepted without an
annoying CMake warning when not actually building a shared libssh2 lib.
Closes#981