Limited Apple OS X support

Worked with Rob Aulwes on getting Byfl to build and run under Apple's OS
X environment.  Some of this effort included copying the latest
Makefiles from llvm/projects/sample.  This necessitated editing
lib/byfl/Makefile to add support for generating bitcode libraries, as
these are apparently no longer supported by the LLVM sample project's
Makefiles.

build-llvm-byfl was modified to work around OS X's lack of "readlink -f"
by providing an abs_dirname() function that replaces the important
missing functionality.

The CC and CXX environment variables are honored if set so users can
override the defaults (gcc and g++, respectively) to point to GCC
versions that work with LLVM, Clang, DragonEgg, and Byfl.

Finally, because OS X currently ships with an ancient bash version
that's unable to run build-llvm-byfl (no coproc support), that script
now explicitly checks BASH_VERSION and aborts with a more user-friendly
message than "syntax error", which is what would happen otherwise.