patch 8.2.4524: MS-Windows: cannot build with some sodium libraries

Problem:    MS-Windows: cannot build with some sodium libraries.
Solution:   Make the DLL name configuragle.  Add build instructions.
            (Ken Takata, closes #9905)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 7b16c59..aaaf530 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -391,7 +391,8 @@
 !if "$(SODIUM)" != "no"
 SOD_INC		= /I "$(SODIUM)\include"
 ! if "$(DYNAMIC_SODIUM)" == "yes"
-SOD_DEFS	= -DHAVE_SODIUM -DDYNAMIC_SODIUM
+SODIUM_DLL	= libsodium.dll
+SOD_DEFS	= -DHAVE_SODIUM -DDYNAMIC_SODIUM -DDYNAMIC_SODIUM_DLL=\"$(SODIUM_DLL)\"
 SOD_LIB		=
 ! else
 SOD_DEFS	= -DHAVE_SODIUM