Detect OS upgrades and refer users to Migration

[Chris Jones]

> On 17 Nov 2013, at 07:22 pm, Landon Fuller <landonf at macports.org> wrote:
> 
> 
>> On Nov 17, 2013, at 14:02 , Chris Jones <jonesc at hep.phy.cam.ac.uk> wrote:
>> 
>> Yes, there are. For instance the change in the default c++ runtime from libstdc++ to libc++ with OSX 10.9. These two runtimes cannot reliably be mixed, so rebuilding all ports is the only safe option. Yes, you might get by not doing this, for a while, but sooner or later you will run into problems, and the root cause is often hard to spot. users might consider the rebuild a pain, but the macPorts devs equally would consider the stream of 'bug' reports because this is not done, a pain... A rebuild is better all round, in the long term.
> 
> Given that macports::revupgrade_scanandrebuild is already scanning for broken dylib dependencies, I think marking C++ runtime mismatches within the scanning dependency graph would be well within its purview?

Possibly, but then i doubt it would spot all such problems (i know for instance it wouldn't spot all problems with root, as there not all libraries are loaded via 'traditional' linker dependencies, but instead via introspection etc. ) plus, i am sure others could come up with other issues, not just the c++ runtime. 

I suppose theoretically macports could perhaps be patched to deal with them all,  but thats a lot of work for someone, and to be honest for me its not clear its worth it. Until such a time, telling users to follow the migration instructions is the best approach.

Chris

> 
> -landonf
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