Various fixes to IE build config

 - Adding app specific compiler flags
 - Adding targetApp specific lib paths
 - Only bulding GafferRenderMan if compiler is valid
Fixing function signature for gcc 4.4
Merge pull request #698 from andrewkaufman/h13

Houdini 13 Compatibility
Added a failing test case that reveals a bug when loading JPG images.
Added a CPP module for GafferImageTest.
Added two tests that compare the OIIO::get_pixel methods against each other to reveal a bug when reading a JPG as Float data.
Fixed GafferImageTest linking os OS X.
Tidied ImageReaderTest includes and namespaces.
Fixed ColorProcessor hashing bug.

It wasn't hashing the channel name into the channelData hash. This had gone unnoticed until now because caching is disabled on that plug, and the OpenColorIO node is the last in the chain in the viewer processing network. Using an OpenColorIO directly in the graph shows the problem immediately.
Fixed ImageReaderTest.testJpgRead

It wasn't taking into account colour space.
Added workaround for OIIO get_pixels() bug.

This fixes the horizontal shift in the green and blue channels when loading a JPEG image.