patch 8.2.4144: cannot load libsodium dynamically

Problem:    Cannot load libsodium dynamically.
Solution:   Support dynamic loading on MS-Windows. (Ken Takata, closes #9554)
diff --git a/src/Make_cyg_ming.mak b/src/Make_cyg_ming.mak
index 4511109..a415631 100644
--- a/src/Make_cyg_ming.mak
+++ b/src/Make_cyg_ming.mak
@@ -668,7 +668,14 @@
 endif
 
 ifeq ($(SODIUM),yes)
+ ifndef DYNAMIC_SODIUM
+DYNAMIC_SODIUM=yes
+ endif
+ ifeq ($(DYNAMIC_SODIUM),yes)
+DEFINES += -DDYNAMIC_SODIUM
+ else
 SODIUMLIB = -lsodium
+ endif
 endif
 
 # Only allow XPM for a GUI build.