fix _POSIX_SYNCHRONIZED_IO usage

it can be -1 for unsupported, or 0 for runtime detection (which we don't
do).

src/drv_maildir.c

@@ -43,7 +43,7 @@
 # define LEGACY_FLOCK 1
 #endif
 
-#ifndef _POSIX_SYNCHRONIZED_IO
+#if !defined(_POSIX_SYNCHRONIZED_IO) || _POSIX_SYNCHRONIZED_IO <= 0
 # define fdatasync fsync
 #endif
 

src/sync.c

@@ -35,7 +35,7 @@
 #include <errno.h>
 #include <sys/stat.h>
 
-#ifndef _POSIX_SYNCHRONIZED_IO
+#if !defined(_POSIX_SYNCHRONIZED_IO) || _POSIX_SYNCHRONIZED_IO <= 0
 # define fdatasync fsync
 #endif