Merge bitcoin/bitcoin#22845: build: improve check for ::(w)system

3ec633e build: improve check for ::(w)system (fanquake)

Pull request description:

  `AC_DEFINE()` takes `HAVE_STD__SYSTEM || HAVE_WSYSTEM` literally, meaning you
  end up with the following in bitcoin-config.h:
  ```cpp
  /* std::system or ::wsystem */
  #define HAVE_SYSTEM HAVE_STD__SYSTEM || HAVE_WSYSTEM
  ```

  This works for the preprocessor, because `HAVE_SYSTEM`, is defined, just unusually. Remove this in favor of setting `have_any_system` in either case, given we don't actually use `HAVE_STD__SYSTEM` or `HAVE_WSYSTEM`, and defining `HAVE_SYSTEM` to 1 thereafter.

ACKs for top commit:
  laanwj:
    Code review ACK 3ec633e

Tree-SHA512: 02c39ba3179136ec1dc28df026b7fa5d732914c85622298ba7ec880f1ae9324208d322a47be451a5c2ff2e165ad1d446bae92e7018db8e517e7ac38fca25a0a3

Author: laanwj

configure.ac

@@ -1229,13 +1229,14 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
  [ AC_MSG_RESULT(no); HAVE_WEAK_GETAUXVAL=0 ]
 )
 
+have_any_system=no
 AC_MSG_CHECKING([for std::system])
 AC_LINK_IFELSE(
     [ AC_LANG_PROGRAM(
         [[ #include <cstdlib> ]],
         [[ int nErr = std::system(""); ]]
     )],
-    [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_STD__SYSTEM, 1, Define to 1 if std::system is available.)],
+    [ AC_MSG_RESULT(yes); have_any_system=yes],
     [ AC_MSG_RESULT(no) ]
 )
 
@@ -1245,11 +1246,13 @@ AC_LINK_IFELSE(
         [[ ]],
         [[ int nErr = ::_wsystem(""); ]]
     )],
-    [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_WSYSTEM, 1, Define to 1 if ::wsystem is available.)],
+    [ AC_MSG_RESULT(yes); have_any_system=yes],
     [ AC_MSG_RESULT(no) ]
 )
 
-AC_DEFINE([HAVE_SYSTEM], [HAVE_STD__SYSTEM || HAVE_WSYSTEM], [std::system or ::wsystem])
+if test "x$have_any_system" != "xno"; then
+  AC_DEFINE(HAVE_SYSTEM, 1, Define to 1 if std::system or ::wsystem is available.)
+fi
 
 LEVELDB_CPPFLAGS=
 LIBLEVELDB=