Merge bitcoin/bitcoin#27696: build: Do not define ENABLE_ZMQ when ZMQ is not available

fa5831b build: Do not define `ENABLE_ZMQ` when ZMQ is not available (Hennadii Stepanov)

Pull request description:

  A new behavior is consistent with the other optional dependencies.

  The source code contains `#if ENABLE_ZMQ` lines only:
  ```
  $ git grep ENABLE_ZMQ -- src/*.cpp
  src/init.cpp:#if ENABLE_ZMQ
  src/init.cpp:#if ENABLE_ZMQ
  src/init.cpp:#if ENABLE_ZMQ
  src/init.cpp:#if ENABLE_ZMQ
  src/init.cpp:#if ENABLE_ZMQ
  ```

  Change in description line -- "Define to 1..." -->  "Define this symbol.." -- is motivated by the fact that the actual value of the defined `ENABLE_ZMQ` macro does not matter at all.

  Related to:
  - bitcoin/bitcoin#16419
  - bitcoin/bitcoin#25302

ACKs for top commit:
  TheCharlatan:
    ACK fa5831b
  jarolrod:
    ACK fa5831b

Tree-SHA512: 5e72ff0d34c4b33205338daea0aae8d7aa0e48fd633e21af01af32b7ddb0532ef68dd3dd74deb2c1d2599691929617e8c09676bcbaaf7d669b88816f866f1db2

Author: fanquake

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

@@ -1632,12 +1632,9 @@ 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 "$use_zmq" = "yes"; then
@@ -1649,6 +1646,8 @@ if test "$use_zmq" = "yes"; then
   esac
 fi
 
+AM_CONDITIONAL([ENABLE_ZMQ], [test "$use_zmq" = "yes"])
+
 dnl libmultiprocess library check
 
 libmultiprocess_found=no
@@ -1843,8 +1842,6 @@ 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