Build failed on OS X Yosemite with clang based g++ (i.e. default g++ installed)

Compiling Qt 5.4.0 on OS X Yosemite is failing with the following error:
clang: error: unsupported option '-gstabs'
which happens as soon as Qt starts to compile qtwebkit.

On Yosemite, g++ is just a wrapper over clang from what I understand and indeed, 'gstabs' doesn't exist for clang.

Seems gstabs flag is set in:
qtwebkit/Tools/qmake/mkspecs/features/unix/default_post.prf
when gcc <= 4.7 (g++ -v mentions 4.2.1)
Commenting the corresponding line fixed the problem.

BUT the thing that really confuses me is: why Qt is using unix/default_post.prf, while there is a mac/default_post.prf? I know OS X is a kind a of UNIX, but as a mac directory exists, I thought this one should be used, and not the other one (i.e. I thought they are mutually exclusive).

Even more confusing for me: I compiled 5.4.0 (same sources) one week ago with similar options, but for 32 bits, and didn't have any problem.
But trying again today doesn't work.

I'm not sure if I simply messed up my environment, or if an Apple update did it for me.

Please let me know if you have any idea what might be wrong, and if I can help you to investigate this issue.