Fix cmake

[pgbarletta]

Hi, I hope a PR is ok.

• XTB_INCLUDE_DIR and XTB_LIBRARY_DIR have to be set as cache variables so the user can modify them.
• The script uses CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT, which came in 3.7.1 and the script's minimum version is 2.8.

Let me know if there's anything to fix.