Fix DESTDIR support for cygwin/msys2 install

fixes #1066

Makefile.install

@@ -91,7 +91,7 @@ ifeq ($(OSNAME), WINNT)
 	@-cp $(LIBDLLNAME).a "$(DESTDIR)$(OPENBLAS_LIBRARY_DIR)"
 endif
 ifeq ($(OSNAME), CYGWIN_NT)
-	@-cp $(LIBDLLNAME) $(OPENBLAS_BINARY_DIR)
+	@-cp $(LIBDLLNAME) "$(DESTDIR)$(OPENBLAS_BINARY_DIR)"
 endif
 endif
 #Generating OpenBLASConfig.cmake