cmake WITH_LIBSODIUM option is broken (#2349)

* cmake WITH_LIBSODIUM option is broken

- Fixed the variable name in platform.hpp.in
- Fixed #if check for randombytes_close() when libsodium is used

* Fixed typo from previous commit

* Reverted compile error fix for randombytes_close()

builds/cmake/platform.hpp.in

@@ -35,7 +35,7 @@
 
 #cmakedefine ZMQ_HAVE_CURVE
 #cmakedefine ZMQ_USE_TWEETNACL
-#cmakedefine HAVE_LIBSODIUM
+#cmakedefine ZMQ_USE_LIBSODIUM
 
 #ifdef _AIX
   #define ZMQ_HAVE_AIX