tde_add_library: Add the ability to link libraries as private.

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>
(cherry picked from commit d504c98bb7)

modules/TDEMacros.cmake

@@ -787,6 +787,7 @@ macro( tde_add_library _arg_target )
   unset( _destination )
   unset( _embed )
   unset( _link )
+  unset( _link_private )
   unset( _dependencies )
   unset( _storage )
   unset( _exclude_from_all )
@@ -881,6 +882,12 @@ macro( tde_add_library _arg_target )
       set( _storage "_link" )
     endif( "+${_arg}" STREQUAL "+LINK" )
 
+    # found directive "LINK_PRIVATE"
+    if( "+${_arg}" STREQUAL "+LINK_PRIVATE" )
+      set( _skip_store 1 )
+      set( _storage "_link_private" )
+    endif( "+${_arg}" STREQUAL "+LINK_PRIVATE" )
+
     # found directive "DEPENDENCIES"
     if( "+${_arg}" STREQUAL "+DEPENDENCIES" )
       set( _skip_store 1 )
@@ -1044,6 +1051,17 @@ macro( tde_add_library _arg_target )
     endif( )
   endif( _embed )
 
+  # set private linked libraries
+  if( _link_private )
+    if( NOT ${CMAKE_VERSION} VERSION_LESS "2.8.12" )
+      if( _link )
+        list( INSERT _link 0 "PUBLIC" )
+      endif()
+      list( APPEND _link "PRIVATE" )
+    endif()
+    list( APPEND _link ${_link_private} )
+  endif( _link_private )
+
   # set link libraries
   if( _link )
     if( _embed AND ${CMAKE_VERSION} VERSION_EQUAL "2.8.12.0" )