if DYLD_FALLBACK_LIBRARY_PATH is empty, use default from man dyld

[nonrational]

I believe this fixes #23

When $DYLD_FALLBACK_LIBRARY_PATH is empty, the default path is used internally. From man dyld

  DYLD_FALLBACK_LIBRARY_PATH
          This is a colon  separated  list  of  directories  that  contain
          libraries.  It is used as the default location for libraries not
          found  in  their  install  path.   By  default,  it  is  set  to
          $(HOME)/lib:/usr/local/lib:/lib:/usr/lib.

By always appending homebrew's lib path to DYLD_FALLBACK_LIBRARY_PATH we end up breaking compatibility between java's ability to correctly reference the default paths for C libs, which breaks, among other things, parts of jruby.

Further Reading:
http://stackoverflow.com/questions/3146274/is-it-ok-to-use-dyld-library-path-on-mac-os-x-and-whats-the-dynamic-library-s#comment21758755_3172515
Homebrew/legacy-homebrew#13463

[gabetax]

Encountered a similar issue with 1.7.12 and 1.7.13, filed issue upstream at jruby/jruby#1830

[dgoodlad]

Shouldn't need to export here

[dgoodlad]

Sorry for the long delay on this. It looks fine except for the small change I noted about not needing to export in your conditional block. Ping me when you've fixed that up and I'll happily merge.

[nonrational]

@dgoodlad doh! good catch. consider it removed.

[dgoodlad]

Released in 1.6.1