Allow user to add flags to CFLAGS and LDFLAGS

This shouldn’t break any existing functionality. The original value of `CFLAGS_ADDTL` now gets passed through to child make tasks. I think. Ha. I’m still new to `make` and `C` and compiling.

This change will allow users running Mac OS X (and any other 64bit UNIX-based OS, I guess) to specify the necessary flags to get `boxes` to build (#7, #9). Here’s what worked for me: `make CFLAGS_ADDTL=-m32 LDFLAGS_ADDTL=-m32`.