Compilation with extrnal LuaJIT broke with addition of FFI

[Kostja Osipov]

Compilation with extrnal LuaJIT broke with addition of FFI: FFI uses lj_obj.h which is not checked for when trying to compile
with external LuaJIT, and in the end tarantool_lua.m fails to compile due to a missing header:
kostja@atlas:~/work/tarantool$ cmake .
...
-- Found a system-wide LuaJIT.
-- Performing Test LUAJIT_RUNS
-- Performing Test LUAJIT_RUNS - Success
-- System-wide LuaJIT at /usr/lib/x86_64-linux-gnu/libluajit-5.1.so is suitable for use.
-- LuaJIT include: /usr/include/luajit-2.0
-- LuaJIT lib: /usr/lib/x86_64-linux-gnu/libluajit-5.1.so
...
kostja@atlas:~/work/tarantool$ make
/home/kostja/work/tarantool/core/tarantool_lua.m:37:20: fatal error: lj_obj.h: No such file or directory
compilation terminated.
make[2]: *** [mod/box/CMakeFiles/ltbox.dir/__/__/core/tarantool_lua.m.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [mod/box/CMakeFiles/ltbox.dir/all] Error 2
make: *** [all] Error 2

[Roman Tsisyk (rtsisyk)]

I recently have same problem on Debian. It seems that Debian does not include lj_*.h headers at all.
Anyway, we should check it in CMake scripts.