Hook to luajit mm (https://github.com/cloudflare/luajit-mm).

To enable it, invoke make-utility with LJMM_DIR=/the/path/to/luajit-mm/dir.

Currently it works only for Linux/x86-64 box. If the target is for
32-bit platform (including i386 family), ljmm is automatically disabled
regardless the LJMM_DIR is specified or not. It might works for
Other-Unix/x86-64. but I don't know for sure at this moment.