Use a relative path for DESTINATION when installing files

[jmcarcell]

BEGINRELEASENOTES

• Use a relative path for DESTINATION when installing files, as recommended by
  the docs (https://cmake.org/cmake/help/latest/command/install.html)

ENDRELEASENOTES

DESTINATION <dir>

    Specify the directory on disk to which a file will be installed. <dir> should be a relative path. An absolute path is allowed, but not recommended.

Nothing should change since it's relative to ${CMAKE_INSTALL_PREFIX}.