https://bugs.exim.org/show_bug.cgi?id=2152#c14
https://vcs.pcre.org/pcre2?view=revision&revision=939

--- pcre2-10.31/src/pcre2grep.c
+++ pcre2-10.31/src/pcre2grep.c
@@ -64,8 +64,8 @@
 #endif
 
 /* Some cmake's define it still */
-#if defined(__CYGWIN__) && !defined(WIN32)
-#define WIN32
+#if defined(__CYGWIN__) && defined(WIN32)
+#undef WIN32
 #endif
 
 #ifdef WIN32