build: use gnulib fdatasync

Commit 1726a736 hacked around MacOS' lack of fdatasync, since
gnulib did not have it at the time.  But now that we use newer
gnulib, we can avoid the hack.

* bootstrap.conf (gnulib_modules): Add fdatasync.
* configure.ac (AC_CHECK_FUNCS_ONCE): Drop our own check.

bootstrap.conf

@@ -40,6 +40,7 @@ environ
 fclose
 fcntl
 fcntl-h
+fdatasync
 ffs
 fnmatch
 fsync

configure.ac

@@ -135,12 +135,9 @@ AC_CHECK_SIZEOF([long])
 
 dnl Availability of various common functions (non-fatal if missing),
 dnl and various less common threadsafe functions
-AC_CHECK_FUNCS_ONCE([cfmakeraw fdatasync geteuid getgid getgrnam_r getmntent_r \
+AC_CHECK_FUNCS_ONCE([cfmakeraw geteuid getgid getgrnam_r getmntent_r \
   getpwuid_r getuid initgroups kill mmap posix_fallocate posix_memalign \
   regexec sched_getaffinity])
-if test $ac_cv_func_fdatasync = no; then
-  AC_DEFINE([fdatasync], [fsync], [Define to fsync if you lack fdatasync])
-fi
 
 dnl Availability of pthread functions (if missing, win32 threading is
 dnl assumed).  Because of $LIB_PTHREAD, we cannot use AC_CHECK_FUNCS_ONCE.