getrandom(2) without GRND_RANDOM is broken prior to Cygwin 2.8.0:

https://sourceware.org/git/?p=newlib-cygwin.git;a=commitdiff;h=6c420fa

--- origsrc/Python-3.6.1/configure.ac	2017-03-21 21:36:53.153779000 -0500
+++ src/Python-3.6.1/configure.ac	2017-03-21 21:42:30.270960000 -0500
@@ -5372,6 +5372,12 @@ AC_MSG_CHECKING(for the getrandom() func
 AC_LINK_IFELSE(
 [
   AC_LANG_SOURCE([[
+    #ifdef __CYGWIN__
+    #include <cygwin/version.h>
+    #if CYGWIN_VERSION_DLL_MAJOR < 2008
+    #error getrandom(2) is broken prior to Cygwin 2.8.0
+    #endif
+    #endif
     #include <sys/random.h>
 
     int main() {