allow building with the system LuaJIT instead of the bundled one

now we can build with the LuaJIT of the system by passing
 -DUSE_BUNDLED_LUAJIT=OFF
or even use an own LuaJIT with
 -DUSE_BUNDLED_LUAJIT=OFF -DLUAJIT_PREFIX=/opt/superluajit

for backwards compatibility, build the bundled version by default
Merge pull request #69 from evgeni/system-luajit

allow building with the system LuaJIT instead of the bundled one. fixes #63