Linkage workaround for macOS in test/clustertest/testplugin applies to Cygwin as well

[wyoung]

I'm not exactly sure how you want to fix this. This works for me:

diff --git a/test/clustertest/testplugin/Makefile b/test/clustertest/testplugin>
index 0c02403..0662c43 100644
--- a/test/clustertest/testplugin/Makefile
+++ b/test/clustertest/testplugin/Makefile
@@ -51,9 +51,9 @@ ifneq ($(MAKECMDGOALS),clean)
 -include $(DEP)
 endif

-# OSX needs to re-link the libraries for some reason
+# Non-Linux linkers usually won't find the libraries implicitly
 UNAME_S := $(shell uname -s)
-ifeq ($(UNAME_S),Darwin)
+ifeq ($(UNAME_S),$(filter $(UNAME_S),Darwin CYGWIN_NT-10.0))
 LIBRARIES = -L../../../ -L../../../mbedtls/library -rdynamic -lpcrecpp -lbedro>
 endif

However, since I've never seen any system but Linux chase linkage implicitly, you might want to flip the sense of the test around, and just check for "not Linux." It'd be simpler, though it might be an over-reach.