autotools: fix Python module file ext for cygwin/msys2

both use .dll, not .pyd
diff --git a/configure.ac b/configure.ac
index c6d1715..a53fc8b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -767,9 +767,12 @@
 PYTHON_LDFLAGS=
 if test "${PYTHON}" != ""; then
     case "$host" in
-        *-*-cygwin* | *-*-mingw* | *-*-msys* )
+        *-*-mingw* )
             PYTHON_LDFLAGS="-no-undefined -shrext .pyd"
             ;;
+        *-*-cygwin* |*-*-msys* )
+            PYTHON_LDFLAGS="-no-undefined"
+            ;;
     esac
 fi
 AC_SUBST(PYTHON_LDFLAGS)