Makefile: allow flags from environment
Makefile: allow flags from environment

Per discussion in #19 (doesn't fix the bug, but will
make it easier to build on OS X without modifying the
Makefile)

BLLIP/bllip-parser,BLLIP/bllip-parser,BLLIP/bllip-parser,BLLIP/bllip-parser,BLLIP/bllip-parser,BLLIP/bllip-parser,BLLIP/bllip-parser,BLLIP/bllip-parser