libunwind/libunwind-headers: use only on < darwin11

[kencu]

the functionality of the libunwind library was added to libSystem in darwin11. Installing this port on any system newer than darwin10 causes problems, and usually causes an older libunwind to be used than the one already in libSystem.

closes: https://trac.macports.org/ticket/66250

For interest:
a current libunwind is currently installed as part of the clang builds, eg.:

% port contents clang-19 | grep libunwind
/opt/local/libexec/llvm-19/include/__libunwind_config.h
/opt/local/libexec/llvm-19/include/libunwind.h
/opt/local/libexec/llvm-19/include/libunwind.modulemap
/opt/local/libexec/llvm-19/lib/libunwind/libunwind.1.0.dylib
/opt/local/libexec/llvm-19/lib/libunwind/libunwind.1.dylib
/opt/local/libexec/llvm-19/lib/libunwind/libunwind.a
/opt/local/libexec/llvm-19/lib/libunwind/libunwind.dylib

if testing a newer libunwind is desired on new systems, one can be found there.

[macportsbot]

Notifying maintainers:
@jeremyhu for port libunwind.

[kencu]

If this is committed, there will be ports discovered that have linked to ${prefix}/lib/libunwind.dylib and will then be broken. These ports will all need to be revbumped.

Some will have overtly had libunwind referenced in their dependencies. Other ports may likely have linked against it opportunistically.

[kencu]

There are ports that look for libunwind, and enable certain functionality if it is found.

On MacOS, that logic will have to be adjusted/changed.

Libunwind is part of libSystem on 10.7+, so looking for a separate libunwind library is pointless. The functionality will have to be tested and enabled a different way.

[edit -- there is in fact libunwind functionality in libSystem 10.6, but Jeremy needed newer stuff to build cctools on 10.6, in the past at least. Nobody has tested whether the libunwind enhancements are still needed on 10.6 to build cctools/etc but I suspect they are needed]

[kencu]

I/we need to verify that cctools will build on 10.7-10.X without the libunwind headers.

If it does, I think this could be pushed and then we start cleaning up the resulting mess of ports that have opportunistically or specifically linked against libunwind.