maint: Correctly detect wether "gluster" cli tool is accessible

The configure definition previously always defined the GLUSTER_CLI macro
and thus the code needing it wasn't compiled out if the tool wasn't
accessible.

configure.ac

@@ -1939,8 +1939,10 @@ AM_CONDITIONAL([WITH_STORAGE_GLUSTER], [test "$with_storage_gluster" = "yes"])
 if test "$with_storage_fs" = "yes" ||
    test "$with_storage_gluster" = "yes"; then
   AC_PATH_PROG([GLUSTER_CLI], [gluster], [], [$PATH:/sbin:/usr/sbin])
+  if test "x$GLUSTER_CLI" != "x"; then
       AC_DEFINE_UNQUOTED([GLUSTER_CLI], ["$GLUSTER_CLI"],
         [Location or name of the gluster command line tool])
+  fi
 fi
 
 LIBPARTED_CFLAGS=