cmake: Follow-up to r1936349, do not use `CMAKE_INSTALL_PREFIX` for the
destinations.
* CMakeLists.txt
(cache variables):
Use variables provided by `GNUInstallDirs` module rather than
`CMAKE_INSTALL_PREFIX` for paths to install bindings.
git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1936532 13f79535-47bb-0310-9956-ffa450edef68