Fix luajit build issues on OSX + refactoring
Fix luajit build issues on OSX + refactoring

Pass sysroot on OSX.
Make the code in luajit.cmake a bit more sane and straightforward.

Sannis/tarantool,Sannis/tarantool,mejedi/tarantool,condor-the-bird/tarantool,KlonD90/tarantool,ocelot-inc/tarantool,KlonD90/tarantool,Sannis/tarantool,Sannis/tarantool,ocelot-inc/tarantool,KlonD90/tarantool,condor-the-bird/tarantool,KlonD90/tarantool,guard163/tarantool,Sannis/tarantool,ocelot-inc/tarantool,KlonD90/tarantool,guard163/tarantool,condor-the-bird/tarantool,guard163/tarantool,rtsisyk/tarantool,condor-the-bird/tarantool,mejedi/tarantool,mejedi/tarantool,ocelot-inc/tarantool,guard163/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,vasilenkomike/tarantool,vasilenkomike/tarantool,vasilenkomike/tarantool,vasilenkomike/tarantool,mejedi/tarantool,vasilenkomike/tarantool,rtsisyk/tarantool,guard163/tarantool,condor-the-bird/tarantool