tde_create_handbook: Install symlink for 'common' part as relative.

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>
(cherry picked from commit 6b4c7a9ef0)

modules/TDEMacros.cmake

@@ -2529,7 +2529,8 @@ macro( tde_create_handbook )
     endif( )
 
     tde_install_empty_directory( ${_dest} )
-    tde_install_symlink( ${TDE_HTML_DIR}/${_lang}/common ${_dest} )
+    file( RELATIVE_PATH _common_link_target "${_dest}" "${TDE_HTML_DIR}/${_lang}/common" )
+    tde_install_symlink( ${_common_link_target} ${_dest} )
 
   endif()