In order to test bedtools after it is installed in /usr/bin, it is needed
to override the BT variable in the shell scripts of the test suite.
The following change achieves it in compliance with POSIX (tested with dash).

-BT=../../bin/bedtools
+BT=${BT-../../bin/bedtools}

If "tac" doesn't exist (e.g., on non GNU/Linux systems like OS X & other BSDs), emulate with sed '1!G;h;$!d' (from http://www.catonmat.net/blog/sed-one-liners-explained-part-one/)

Since the Makefile executes tests using "sh" instead of "bash", use the more portable "> obs 2>&1" instead of "&> obs" so the test will pass when /bin/sh isn't bash (e.g., on Debian-based Linux distributions, and most non-Linux *nix systems).

bedtools subcommands.