build: Do not define `ENABLE_ZMQ` when ZMQ is not available

A new behavior is consistent with the other optional dependencies.

The source code contains `#if ENABLE_ZMQ` lines only.

build_msvc/bitcoin_config.h.in

@@ -38,7 +38,7 @@
 /* Define to 1 to enable SQLite wallet */
 #define USE_SQLITE 1
 
-/* Define to 1 to enable ZMQ functions */
+/* Define this symbol to enable ZMQ functions */
 #define ENABLE_ZMQ 1
 
 /* define if external signer support is enabled (requires Boost::Process) */

configure.ac

@@ -1633,12 +1633,9 @@ dnl ZMQ check
 
 if test "$use_zmq" = "yes"; then
   PKG_CHECK_MODULES([ZMQ], [libzmq >= 4],
-    AC_DEFINE([ENABLE_ZMQ], [1], [Define to 1 to enable ZMQ functions]),
-    [AC_DEFINE([ENABLE_ZMQ], [0], [Define to 1 to enable ZMQ functions])
-    AC_MSG_WARN([libzmq version 4.x or greater not found, disabling])
+    AC_DEFINE([ENABLE_ZMQ], [1], [Define this symbol to enable ZMQ functions]),
+    [AC_MSG_WARN([libzmq version 4.x or greater not found, disabling])
     use_zmq=no])
-else
-  AC_DEFINE_UNQUOTED([ENABLE_ZMQ], [0], [Define to 1 to enable ZMQ functions])
 fi
 
 if test "$use_zmq" = "yes"; then
@@ -1650,6 +1647,8 @@ if test "$use_zmq" = "yes"; then
   esac
 fi
 
+AM_CONDITIONAL([ENABLE_ZMQ], [test "$use_zmq" = "yes"])
+
 dnl libmultiprocess library check
 
 libmultiprocess_found=no
@@ -1844,8 +1843,6 @@ if test "$bitcoin_enable_qt" != "no"; then
   fi
 fi
 
-AM_CONDITIONAL([ENABLE_ZMQ], [test "$use_zmq" = "yes"])
-
 AC_MSG_CHECKING([whether to build test_bitcoin])
 if test "$use_tests" = "yes"; then
   if test "$enable_fuzz" = "yes"; then