LuaJIT is 5.1-compatible and is maintained unlike lua5.1 itself, so
try to allow it.

Use the eclass' created unversioned lua.pc, and define the compat
macro that is normally defined in 5.1's lauxlib.h but not luajit.

TODO: an upstreamable fix involve find_package for lua (can manipulate
preferences with cmake args), and no longer using luaL_reg at all.
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -186,3 +186,3 @@
   # Check for LUA
-  pkg_search_module( LUA lua5.1 lua-5.1 )
+  pkg_search_module( LUA lua )
 
--- a/src/lua-engine.cpp
+++ b/src/lua-engine.cpp
@@ -124,2 +124,5 @@
 #include <lauxlib.h>
+#ifndef luaL_reg
+#define luaL_reg luaL_Reg
+#endif
 #include <lualib.h>