build: improve check for ::(w)system

`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 defining `HAVE_SYSTEM` to 1 in either case, given we
don't actually use `HAVE_STD__SYSTEM` or `HAVE_WSYSTEM`. We just use ::system if
we aren't building for Windows.

Author: fanquake

configure.ac

@@ -1245,13 +1245,14 @@ 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) ]
 )
 
@@ -1261,11 +1262,13 @@ 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=