Upstream build scripts use either system LuaJIT or bundled PUC Lua 5.1.
Make it possible to use the system version of the latter too.

--- a/cmake/Modules/FindLua.cmake
+++ b/cmake/Modules/FindLua.cmake
@@ -2,27 +2,12 @@
 # This selects LuaJIT by default
 
 option(ENABLE_LUAJIT "Enable LuaJIT support" TRUE)
-set(USE_LUAJIT FALSE)
-option(REQUIRE_LUAJIT "Require LuaJIT support" FALSE)
-if(REQUIRE_LUAJIT)
-	set(ENABLE_LUAJIT TRUE)
-endif()
 if(ENABLE_LUAJIT)
-	find_package(LuaJIT)
-	if(LUAJIT_FOUND)
-		set(USE_LUAJIT TRUE)
-		message (STATUS "Using LuaJIT provided by system.")
-	elseif(REQUIRE_LUAJIT)
-		message(FATAL_ERROR "LuaJIT not found whereas REQUIRE_LUAJIT=\"TRUE\" is used.\n"
-			"To continue, either install LuaJIT or do not use REQUIRE_LUAJIT=\"TRUE\".")
-	endif()
+	find_package(LuaJIT REQUIRED)
+	set(USE_LUAJIT TRUE)
+	message (STATUS "Using LuaJIT provided by system.")
 else()
-	message (STATUS "LuaJIT detection disabled! (ENABLE_LUAJIT=0)")
-endif()
-
-if(NOT USE_LUAJIT)
-	message(STATUS "LuaJIT not found, using bundled Lua.")
-	set(LUA_LIBRARY lua)
-	set(LUA_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/lib/lua/src)
-	add_subdirectory(lib/lua)
+	find_package(Lua51 REQUIRED)
+	set(USE_LUAJIT FALSE)
+	message(STATUS "Using Lua5.1 provided by system.")
 endif()