configure: remove unused HAVE_FDATASYNC

configure.ac

@@ -600,7 +600,7 @@ AC_CHECK_DECLS([nanosleep],,,[#include <time.h>])
 AC_CHECK_FUNCS([daemon fcntl flock fstatvfs fork getenv getpwuid_r isatty lstat memalign mkostemp mmap open_memstream openat pread posix_fadvise posix_madvise setlocale stricmp strnicmp strptime uselocale pthread_cond_timedwait_monotonic_np pthread_condattr_setclock])
 AC_REPLACE_FUNCS([atof atoll dirfd fdopendir ffsll flockfile fsync getdelim getpid lldiv memrchr nrand48 poll posix_memalign recvmsg rewind sendmsg setenv strcasecmp strcasestr strdup strlcpy strndup strnlen strnstr strsep strtof strtok_r strtoll swab tdestroy timegm timespec_get strverscmp])
 AC_REPLACE_FUNCS([gettimeofday])
-AC_CHECK_FUNCS(fdatasync,,
+AC_CHECK_FUNC(fdatasync,,
   [AC_DEFINE(fdatasync, fsync, [Alias fdatasync() to fsync() if missing.])
 ])