btrfs-progs: autogen: Don't show success message on fail

btrfs-progs: autogen: Don't show success message on fail

When autogen.sh failed, the success message is still in output:
 # ./autogen.sh
 ...
 configure.ac:131: error: possibly undefined macro: PKG_CHECK_VAR
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.

 Now type './configure' and 'make' to compile.
 #

Fixed by check return value of autoconf.

After patch:
 # ./autogen.sh
 ...
 configure.ac:132: error: possibly undefined macro: PKG_CHECK_VAR
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
 #

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>