gazebo5: fix gdal build issue
gazebo5: fix gdal build issue

There has been an issue with building gazebo with gdal
support on osx (see the following issue:
https://bitbucket.org/osrf/gazebo/issue/1063 )
This commit patches gazebo5 with a fix and adds
gdal as an optional dependency.

scpeters/homebrew-simulation,NikolausDemmel/homebrew-simulation,hgaiser/homebrew-simulation,j-rivero/homebrew-simulation,osrf/homebrew-simulation