cmake: define LUA_INCLUDE_DIR

Follows up the commit 2a58b8a
("rockspec: propagate CMake options").

Author: ligurio

CMakeLists.txt

@@ -23,6 +23,7 @@ if(CMAKE_LUA_INCLUDE_DIR AND CMAKE_LUA_LIBRARIES)
   add_library(liblua STATIC IMPORTED GLOBAL)
   set_target_properties(liblua PROPERTIES
     IMPORTED_LOCATION ${CMAKE_LUA_LIBRARIES})
+  set(LUA_INCLUDE_DIR ${CMAKE_LUA_INCLUDE_DIR})
   set(LUA_LIBRARIES liblua)
   set(LUA_FOUND TRUE)
 elseif(ENABLE_LUAJIT)