Fixed linking on OS X.

The OS X linker checks everything at link time rather than deferring to load time like the linux linker - so we have to explicitly link in more libraries.
Added missing declaration for GCC 4.2.1.
Updated linking for OS X.
Update JH build to use 3delight 10.0.127.
Added rudimentary Gaffer.app bundle for OS X.

Still needs an icon, and packaging as a .dmg file for installation.
Set up DL_DISPLAYS_PATH and DL_PROCEDURALS_PATH automatically in GafferRenderMan module.
Updated 3delight version.
Fixed IECOREGL_SHADER_PATHS setup in wrapper.
Improved speed of renderman shader menu.

The 3delight installer was adding some dubious paths, and making duplicate paths too - see comments.
Added support for packaging as .dmg on OS X.
Merge pull request #366 from johnhaddon/macBuildFixes

Mac build fixes