CMake refactoring

Externals/CMakeLists.txt

@@ -1,11 +1,25 @@
-add_subdirectory(LuaJIT-proj)
+if (XRAY_USE_LUAJIT)
+    add_subdirectory(LuaJIT-proj)
+else()
+    find_package(Lua51 REQUIRED)
+endif()
+
 add_subdirectory(xrLuaFix)
 add_subdirectory(luabind)
+
 if (USE_CXIMAGE)
     add_subdirectory(cximage)
 endif()
+
 add_subdirectory(GameSpy)
 add_subdirectory(OPCODE)
 add_subdirectory(ode)
 #add_subdirectory(NVTT)
 add_subdirectory(imgui-proj)
+
+if (NOT TARGET xrLuabind)
+    message(FATAL_ERROR
+        "You probably have downloaded OpenXRay sources as ZIP archive, you can't do that. Use git to clone the repository.\n"
+        "Read the build instructions: https://github.com/OpenXRay/xray-16/wiki"
+    )
+endif()