install: Configurable DESTINATION for the cmake.m4 file

The DESTINATION changed from share/aclocal to ${CMAKE_XDGDATA_DIR}/aclocal.

Auxiliary/CMakeLists.txt

 install(DIRECTORY vim/indent vim/syntax DESTINATION ${CMAKE_DATA_DIR}/editors/vim)
 install(FILES cmake-mode.el DESTINATION ${CMAKE_DATA_DIR}/editors/emacs)
-install(FILES cmake.m4 DESTINATION share/aclocal)
+install(FILES cmake.m4 DESTINATION ${CMAKE_XDGDATA_DIR}/aclocal)
 add_subdirectory (bash-completion)