Adjusting to compile in OS X.

Some little fixes to make it compile on OS X 10.9.

````
➜  luadata git:(darwin) make -f GNUMakefile clean && make -f GNUMakefile test
rm -f *.so *.o test || true
gcc -I. -fPIC   -c -o luadata.o luadata.c
gcc -I. -fPIC   -c -o data.o data.c
gcc -I. -fPIC   -c -o layout.o layout.c
gcc -I. -fPIC   -c -o binary.o binary.c
gcc -I. -fPIC   -c -o luautil.o luautil.c
gcc -I. -fPIC   -c -o luacompat502.o luacompat502.c
gcc -shared -o data.so luadata.o data.o layout.o binary.o luautil.o luacompat502.o -llua
gcc -I. -fPIC    test.c data.so  -llua -o test
➜  luadata git:(darwin) ./test 
test passed ;-)
````