b43-fwcutter: 018 - extract Broadcom firmware

Part of the b43-tools package, this utility analyzes vendor-provided
drivers and extracts the proprietary firmware necessary for the Linux
b43 module to initialize Broadcom wireless chips.

This formula patches b43-fwcutter so that it builds on Mac OS. The
patch adds preprocessor directives to use Apple's xnu headers in favor
of GNU's `byteswap.h` This patch has been reported and accepted
upstream, but has not yet been officially released. See
<https://github.com/mbuesch/b43-tools/pull/1>.

This formula also patches the Makefile to honor Homebrew's
installation and manpage prefixes, since this tool has no configure
script.