special cases:

* change systemd detection to use CHECK_LIBRARY_EXISTS at least once,
  to have it detected by build_depends.cmake
* similarly, use find_library for pam
* unixODBC is weird, libodbc.so is in the unixODBC package, not
  in the unixODBC-devel, where normally all .so files belong.
  Packaging bug? As a workaround, use find_file(sql.h) instead of
  find_path(sql.h) to make sure that /usr/include/sql.h (not /usr/include)
  is cached by cmake, and later build_depends.cmake will select
  unixODBC-devel, as a package owning /usr/include/sql.h file.