Fix linking against serf on some platforms.
Fix linking against serf on some platforms.

* build/ac-macros/serf.m4: add serf's libdir to LDFLAGS

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion