Support Xcode project creation with waf

[jpauwels]

A PR that might be mostly beneficial for me, but it's been living for years in my fork and has no drawbacks, so I thought I'd share in case anyone else uses macOS.

It is possible to let waf generate Xcode project files (so you can do graphical debugging) by adding the necessary syntax and including the relevant extension into the waf copy in the repo. You just need to remember to include the various extensions (swig, emscripten and now xcode6 too) when updating waf otherwise you end up breaking something somewhere, so I made a little script to help with future updates.

I've made similar changes to Essentia, which I'll post as soon as my summer-clean of forks gets there :-)

[jpauwels]

Forgot to document how to use it: waf configure --with-python-bindings --with-tests xcode6 then open gaia.xcodeproj in build/.