Cyclic dependency between <sys/ucontext.h> and <ucontext.h>

[Andreas Schwab]

There is a cyclic dependency between the headers <sys/ucontext.h> and
<ucontext.h> when _XOPEN_SOURCE is defined to 500 or 600: <sys/ucontext.h>
includes <signal.h> which includes <ucontext.h> which includes
<sys/ucontext.h>.  The effect is that you'll get a compiler error when
including <sys/ucontext.h> due to ucontext_t begin undefined.  I don't
know how to fix that properly, a workaround is to include <ucontext.h>
instead.

Andreas.

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Andreas Schwab, SuSE Labs, schwab@suse.de
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