Portability fixes for sigwait.

Build farm animals running ancient HPUX and Solaris have a non-standard
sigwait() from draft versions of POSIX, so they didn't like commit
7c09d279.  To avoid the problem in general, only try to use sigwait() if
it's declared by <signal.h> and matches the expected declaration.  To
select the modern declaration on Solaris (even in non-threaded
programs), move -D_POSIX_PTHREAD_SEMANTICS into the right place to
affect all translation units.

Also fix the error checking.  Modern sigwait() doesn't set errno.

Thanks to Tom Lane for help with this.

Discussion: https://postgr.es/m/3187588.1626136248%40sss.pgh.pa.us

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