Make our CMake config more self-documenting by introducing variables
for the shared and static lib target names. Without this, it might be
non-trivial to find out which line is referring to a target name vs
libname, export name or other occurrences of `libssh2`.
This allows to rename back the shared lib target name to the value used
before 4e2580628d:
`libssh2_shared` -> `libssh2`, if necessary for compatibility. Notice:
before that patch, `libssh2` name referred to either the static or
shared lib, depending on build settings.
5a4d532ce5