AX_CHECK_LIBRARY: Bump serial to match upstream

The m4_ifnblank patch was accepted upstream as
http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=commitdiff;h=7ea1d1b18e5c1c3300936dd89c3e55b5b370ae72



Signed-off-by: Anders Kaseorg <andersk@mit.edu>

(closes #153)