[Bug 38713] Link failure due to use of libunwind in ntdll

[wine-bugs at winehq.org]

https://bugs.winehq.org/show_bug.cgi?id=38713

--- Comment #6 from Ken Thomases <ken at codeweavers.com> ---
Created attachment 51676
  --> https://bugs.winehq.org/attachment.cgi?id=51676
configure check for libunwind

(In reply to Erich E. Hoover from comment #3)

> Created attachment 51673 [details]
> ntdll: Link to libunwind on x86-64 Linux.

Thanks for this.

> Please try the attached on Mac :)  I used UNWIND_LIBS instead of
> LIBUNWIND_LIBS since that seems to match the current convention for these
> libraries.

Hmm.  Well, on the Mac it's mostly harmless.  There is no separate library for
the libunwind functionality.  It's present in the C library that's linked by
default.  So, your check doesn't find any library and the link goes fine.

However, it does emit an unnecessary notice.

I think that checking for _Unwind_RaiseException probably is a good solution to
the symbol renaming that the header does for things like unw_init_local(). 
With that, we can use AC_SEARCH_LIBS, which I think is the more appropriate
checking macro.  It explicitly supports the possibility that no library may be
necessary for the target symbol.  I'm attaching an alternative patch using that
approach.

It adds a new config.h define, HAVE_LIBUNWIND, and changes the code in
signal_x86_64.c to test that.  So, the code is only used if both the header and
the symbol are available.

I've eliminated the notice.  If libunwind is not available, I'm not sure that's
a problem that the user/builder/packager needs to address.  It will just go
back to the situation before my commit, which was mostly fine.  The only
problem (that I'm aware of) was on OS X, where there's not really any question
that libunwind will be available.  If desired, we can add the notice back based
on the variable ac_cv_search__Unwind_RaiseException.

Can you test on x86-64 Linux?  Thanks again.

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