Does it really not build anymore on 10.5, or is it just that we stopped supporting it in the released binaries?

The gitian build uses 10.7 (see https://github.com/bitcoin/bitcoin/blob/master/doc/release-process.md#fetch-and-build-inputs-first-time-or-when-dependency-versions-change).
Maybe it would be worth to define what min OSX version we support (i would set 10.7 as it is the gitians base).

I'm not sure about this pull. Maybe someone likes to build it with 10.6 for any reason.

10.6 is the current minimum for 0.10. Gitian builds with -mmacosx-version-min=10.6 regardless of which sdk is used. Fwiw, this is confirmed working with SDKs up to 10.9, I haven't tried 10.10.

Which versions should be supported is a separate discussion for after 0.10 is released.

@theuni Ah. Right. We build against 10.7 but use 10.6 as minimum target which mainly set some macros, etc.

ACK on this pull.

ACK from me too, but let's make it explicit. If an sdk is too old to build with, it should fail to configure.

I think it should suffice to add a compile-test like

#include Availability.h
#if __MAC_OS_X_VERSION_MIN_REQUIRED < 1060
fail()
#endif
int main(){return 0;}

That should fail to compile on old SDKs, or if the compat version is set <= 10.6

I prefer making the old OSX version fail explictly or no action. (Unsupported doesn't mean we should go break things; but if we do break things better to make it explicit when we know it won't work.)

Agreed @gmaxwell @theuni . I just meant to say that the version that gitian builds against is not necessarily the minimum version that people could build manually against if they wanted. So there are different levels of "support" in play here. At least for Linux it is important that Bitcoin Core can stil be built on older, stable server distributions. For MacOSX this is arguably less of an issue.