build: Detect the appropriate default flavour
build: Detect the appropriate default flavour

If we're building on/for Windows, set 'win32' as the default flavour; if
we're building on OS X, set 'osx' as the default flavour. For everything
else, use 'glx'.

GNOME/clutter,jigpu/clutter,djdeath/clutter-android,collects/clutter,spatulasnout/clutter,djdeath/clutter-android,heysion/clutter-clone,jigpu/clutter,jigpu/clutter,Distrotech/clutter,jigpu/clutter,djdeath/clutter,Distrotech/clutter,Distrotech/clutter,jigpu/clutter,collects/clutter,Distrotech/clutter,ebassi/clutter,kerrickstaley/clutter-vala,ebassi/clutter,heysion/clutter-clone,heysion/clutter-clone,kerrickstaley/clutter-vala,djdeath/clutter-android,ebassi/clutter,djdeath/clutter,djdeath/clutter-multithreaded,djdeath/clutter,djdeath/clutter,Distrotech/clutter,djdeath/clutter-multithreaded,GNOME/clutter,spatulasnout/clutter,collects/clutter,ebassi/clutter,GNOME/clutter,spatulasnout/clutter,djdeath/clutter-android,GNOME/clutter,spatulasnout/clutter,spatulasnout/clutter,heysion/clutter-clone,jigpu/clutter,djdeath/clutter-multithreaded,kerrickstaley/clutter-vala,kerrickstaley/clutter-vala,collects/clutter,Distrotech/clutter,djdeath/clutter,djdeath/clutter-android,heysion/clutter-clone,collects/clutter,djdeath/clutter-multithreaded,ebassi/clutter,GNOME/clutter,djdeath/clutter