Merge #19803: Bugfix: Define and use HAVE_FDATASYNC correctly outside LevelDB

c4b85ba704 Bugfix: Define and use HAVE_FDATASYNC correctly outside LevelDB (Luke Dashjr)

Pull request description:

  Fixes a bug introduced in #19614

  The LevelDB-specific fdatasync check was only using `AC_SUBST`, which works for Makefiles, but doesn't define anything for C++. Furthermore, the #define is typically 0 or 1, never undefined.

  This fixes both issues by defining it and checking its value instead of whether it is merely defined.

  Pulled out of #14501 by fanquake's request

ACKs for top commit:
  fanquake:
    ACK c4b85ba704 - thanks for catching and fixing my mistake.
  laanwj:
     Code review ACK c4b85ba704

Tree-SHA512: 91d5d426ba000b4f3ee7e2315635e24bbb23ceff16269ddf4f65a63d25fc9e9cf94a3b236eed2f8031cc36ddcf78aeb5916efcb244f415943a8a12f907ede8f9

src/util/system.cpp

@@ -1019,7 +1019,7 @@ bool FileCommit(FILE *file)
         return false;
     }
 #else
-    #if defined(HAVE_FDATASYNC)
+    #if HAVE_FDATASYNC
     if (fdatasync(fileno(file)) != 0 && errno != EINVAL) { // Ignore EINVAL for filesystems that don't support sync
         LogPrintf("%s: fdatasync failed: %d\n", __func__, errno);
         return false;