os x build fix

[szagoruyko]

fixes #5

[ksarma]

This worked for me, thanks!

[infoshoc]

This actually fixed my linking error on os x:

Undefined symbols for architecture x86_64:
"_luaL_error", referenced from:
_lstop in mattorchlive.c.o
"_luaL_loadbuffer", referenced from:
_mattorch_dostring in mattorchlive.c.o
"_luaL_loadfile", referenced from:
_mattorch_dofile in mattorchlive.c.o
"_luaL_newstate", referenced from:
_mattorch_init in mattorchlive.c.o
"_luaL_openlibs", referenced from:
_mattorch_init in mattorchlive.c.o
"_lua_call", referenced from:
_mattorch_callfunc in mattorchlive.c.o
_traceback in mattorchlive.c.o
"_lua_close", referenced from:
_mattorch_close in mattorchlive.c.o
"_lua_executable_dir", referenced from:
_mattorch_init in mattorchlive.c.o
"_lua_gc", referenced from:
_mattorch_init in mattorchlive.c.o
_docall in mattorchlive.c.o
"_lua_getfield", referenced from:
_mattorch_dorequire in mattorchlive.c.o
_mattorch_callfunc in mattorchlive.c.o
_traceback in mattorchlive.c.o
"_lua_gettop", referenced from:
_docall in mattorchlive.c.o
"_lua_insert", referenced from:
_docall in mattorchlive.c.o
"_lua_isstring", referenced from:
_traceback in mattorchlive.c.o
"_lua_pcall", referenced from:
_docall in mattorchlive.c.o
"_lua_pushcclosure", referenced from:
_docall in mattorchlive.c.o
"_lua_pushinteger", referenced from:
_traceback in mattorchlive.c.o
"_lua_pushstring", referenced from:
_mattorch_dorequire in mattorchlive.c.o
"_lua_pushvalue", referenced from:
_traceback in mattorchlive.c.o
"_lua_remove", referenced from:
_docall in mattorchlive.c.o
"_lua_sethook", referenced from:
_laction in mattorchlive.c.o
_lstop in mattorchlive.c.o
"_lua_settop", referenced from:
_report in mattorchlive.c.o
_mattorch_callfunc in mattorchlive.c.o
_traceback in mattorchlive.c.o
"_lua_tolstring", referenced from:
_report in mattorchlive.c.o
"_lua_type", referenced from:
_report in mattorchlive.c.o
_traceback in mattorchlive.c.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [libmattorchlive.dylib] Error 1
make[1]: *** [CMakeFiles/mattorchlive.dir/all] Error 2
make: *** [all] Error 2