Update installation instructions.

[benjamin-heasly]

A recent attempt to install RenderToolbox3 on a fresh OS X machine revealed that the instructions are out of date. See wiki pages:

• Installing-RenderToolbox3
• Pre-Built-Renderers
• Building Renderers

[benjamin-heasly]

Some specifics from David and Nicolas:

• The KyngChaos installer for gsl didn’t work under the newer OS I was running (Yosemete or El Capitan). But Homebrew “brew install gal” worked and then PBRT ran.
• We also had the usual problems compiling ReadMultichannelEXR. Nicolas has a solution where he puts a mexopts.sh file in the same directory as the source — apparently this is where Matlab looks first. This worked, while following your instructions nor the instructions for 2015b on the Mathworks site both failed. I am trying to check in that version of mexopts.sh, but at the moment am struggling with git authorization issues on that machine.
• In following the instructions, I did not find it immediately apparent where to put the pre-compiled PBRT binary, that might use a little tweaking.
• The local mexopts.sh that was just pushed to github will work for 10.11 (ElCapitan). For earlier systems, all the references to 10.11 SDK should be changed accordingly, i.e., 10.10 for Yosemite, 10.9 for Mavericks.

[benjamin-heasly]

For Version 3, we will have several new dependencies, and a new Archiva server from which we can download renderer binaries.

So we will be updating the install instructions.

[benjamin-heasly]

Closing this issue. It should be addressed in RenderToolbox4: https://github.com/RenderToolbox/RenderToolbox4