Use DYLD_FALLBACK_LIBRARY_PATH to not interfere with system libraries on OS X.

I'm currently working on creating a set of Portfiles to get ROS properly supporting within macports (https://github.com/kyonifer/ros-macports). One issue i've run into is that ROS uses DYLD_LIBRARY_PATH to find libs at runtime. In general, this approach causes issues with DYLD_ libraries overriding builtin libraries. For example, once a user has sourced setup.bash, macport's libjpeg overrides the system libJPEG which export different symbols. This makes it impossible to run e.g. Sublime Text or rqt:
```
kyons-MacBook-Pro:~ kyon$ source setup.bash
kyons-MacBook-Pro:~ kyon$ sl
dyld: Symbol not found: __cg_jpeg_resync_to_restart
  Referenced from: /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
  Expected in: /opt/local/lib/libJPEG.dylib
 in /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
Trace/BPT trap: 5
kyons-MacBook-Pro:~ kyon$ 
kyons-MacBook-Pro:~ kyon$ rqt
Traceback (most recent call last):
  File "/opt/local/bin/rqt", line 16, in <module>
    sys.exit(main.main())
  File "/opt/local/lib/python2.7/site-packages/rqt_gui/main.py", line 56, in main
    return super(Main, self).main(argv, standalone=standalone, plugin_argument_provider=plugin_argument_provider)
  File "/opt/local/lib/python2.7/site-packages/qt_gui/main.py", line 326, in main
    from python_qt_binding import QT_BINDING
  File "/opt/local/lib/python2.7/site-packages/python_qt_binding/__init__.py", line 45, in <module>
    from .binding_helper import loadUi, QT_BINDING, QT_BINDING_MODULES, QT_BINDING_VERSION  # @UnusedImport
  File "/opt/local/lib/python2.7/site-packages/python_qt_binding/binding_helper.py", line 258, in <module>
    _select_qt_binding(getattr(sys, 'SELECT_QT_BINDING', None))
  File "/opt/local/lib/python2.7/site-packages/python_qt_binding/binding_helper.py", line 84, in _select_qt_binding
    raise ImportError('Could not find Qt binding (looked for "%s"):\n%s' % (DEFAULT_BINDING_ORDER, '\n'.join(error_msgs)))
ImportError: Could not find Qt binding (looked for "['pyqt', 'pyside']"):
  ImportError for "pyqt": dlopen(/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/PyQt4/QtCore.so, 2): Symbol not found: __cg_jpeg_resync_to_restart
  Referenced from: /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
  Expected in: /opt/local/lib/libJPEG.dylib
 in /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
  ImportError for "pyside": No module named PySide.QtCore
```

Theres two solutions to this: 

* Ask libraries to use install names with full paths (or rpaths) encoded, thereby eliminating libs from being on the path at all. 
* Use DYLD_FALLBACK_LIBRARY_PATH, as dyld will only search the fallback path once it exhausts the builtin paths (eliminating interference with links that were previously working). 

The latter is easily changed as seen in the attached pull request. The former is done by passing flags to catkin_make while building catkin and then eliminating DYLD_ vars from the setup.bash altogether. See the ros-desktop Portfile in the above repository for an example.

I've done both for macports, but it'd be good to change things upstream one way or the other.