Issue #21085: add configure check for siginfo_t.si_band

Patch by Masayuki Yamamoto, reviewed and rebased by Erik Bray.

This is a first step on the long road toward resupporting Cygwin, which does
not provide siginfo_t.si_band.
diff --git a/Misc/ACKS b/Misc/ACKS
index a5465a2..467f3aa 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1671,6 +1671,7 @@
 Arnon Yaari
 Alakshendra Yadav
 Hirokazu Yamamoto
+Masayuki Yamamoto
 Ka-Ping Yee
 Chi Hsuan Yen
 Jason Yeo
diff --git a/Misc/NEWS b/Misc/NEWS
index 8a0dac9..9d48ff8 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -173,6 +173,9 @@
 Build
 -----
 
+- Issue #21085: Add configure check for siginfo_t.si_band, which Cygwin does
+  not provide.  Patch by Masayuki Yamamoto with review and rebase by Erik Bray.
+
 - Issue #28258: Fixed build with Estonian locale (python-config and distclean
   targets in Makefile).  Patch by Arfrever Frehtes Taifersar Arahesis.