configure.ac: fix HAVE_SYNC_FILE_RANGE definition

If sync_file_range is not available when building the configure script,
there is a cosmetic bug when running that script reporting
"HAVE_SYNC_FILE_RANGE: command not found".  Remove that error message by
defining HAVE_SYNC_FILE_RANGE to an empty string, rather than generating
a script where that appears as a bare command.

Signed-off-by: Adam Dinwoodie <adam@dinwoodie.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

configure.ac

@@ -1087,7 +1087,7 @@ GIT_CONF_SUBST([HAVE_CLOCK_MONOTONIC])
 # Define HAVE_SYNC_FILE_RANGE=YesPlease if sync_file_range is available.
 GIT_CHECK_FUNC(sync_file_range,
 	[HAVE_SYNC_FILE_RANGE=YesPlease],
-	[HAVE_SYNC_FILE_RANGE])
+	[HAVE_SYNC_FILE_RANGE=])
 GIT_CONF_SUBST([HAVE_SYNC_FILE_RANGE])
 
 #