fa5831bd6f 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:
- https://github.com/bitcoin/bitcoin/issues/16419
- https://github.com/bitcoin/bitcoin/pull/25302
ACKs for top commit:
TheCharlatan:
ACK fa5831bd6f
jarolrod:
ACK fa5831bd6f
Tree-SHA512: 5e72ff0d34c4b33205338daea0aae8d7aa0e48fd633e21af01af32b7ddb0532ef68dd3dd74deb2c1d2599691929617e8c09676bcbaaf7d669b88816f866f1db2
We already use a mix of <cstdio> and stdio.h unconditionally throughout
the codebase.
Us checking this header also duplicates work already done by autotools.
Currently stdio.h is checked for 3 times during a ./configure run, after
this change, at least it's only twice.
We don't include strings.h anywhere.
This is also already checked for by autoconf, so us checking for it just
means a 3rd existence check during ./configure.
