Working on Mac support

Trying, with Rob Aulwes, to get Byfl to build and run under Apple OS X.
Updated Byfl build system

Copied the latest Makefiles from llvm/projects/sample and edited
lib/byfl/Makefile to add support for generating bitcode libraries, as
these are apparently no longer supported by the LLVM sample project's
Makefiles.
Honor CC and CXX variables

Made the install script use the CC and CXX environment variables if
set.  If not, gcc and g++ are used instead.
Minimum bash version

Because build-llvm-byfl uses bash's coproc statement, the script
doesn't work with versions of bash prior to 4.0 as these lack coproc
support.  Alas, Apple ships a 3.x version of bash with the current
OS X, which means the script can't run on a "pure" OS X
installation.  As a stopgap measure, the script now explicitly
checks BASH_VERSION and aborts with a more user-friendly message
than "syntax error", which is what would happen otherwise.
Attempt to fix threading issues

This was an attempt to improve the way Byfl handles multithreaded
code.  So far, it still doesn't work correctly with LGNCG.
Wrote a bfbin2sqlite3 man page as reStructuredText
Override wrapped compilers with environment variables

I just upgraded my development machine from Ubuntu 14.04 to Ubuntu
14.10, Doing so upgraded the compiler from GCC 4.8 to GCC 4.9.  I then
discovered, to my chagrin, that DragonEgg doesn't build against GCC
4.9.  (This is with LLVM/Clang/DragonEgg revision 223694.)

This revision of Byfl therefore includes two changes to address this
issue: (1) it produces an error message is build-llvm-byfl is run
against GCC 4.9, and (2) it enables the BF_GCC, BF_GXX, BF_GFORTRAN,
and BF_GCCGO environment variables to override the underlying compiler
used by, respectively, bf-gcc, bf-g++, bf-gfortran, and bf-gccgo.
Merge branch 'mac-support'
Merge branch 'rst-man-pages'
Merge branch 'thread-fixes'