When ENABLE_PKCS11 is not defined (probably my fault, will investigate separately), openssh will fail to build because the #ifdef in ssh-pkcs11-helper.c skips the part that #includes log.h, which defines SYSLOG_LEVEL_ERROR and SYSLOG_FACILITY_AUTH. Those (and the prototype for log_init) are needed on line 369.
I found out why my build did not define ENABLE_PKCS11. I was building a statically linked binary with my embedded libc. There is another, related bug. ssh-pkcs11-helper fails to build in the above configuration, because it has an undefined reference to xstrdup. That is used by bsd-misc.o but not linked into openbsd-compat.a. When I add xmalloc.o manually to openbsd-compat.a, the build works.
Thanks for the report; fixes to both issues (bad #ifdef order and broken -lssh dep) have been committed and will be in the next release. For the xstrdup thing, I just appended -lssh to the link line - we already do this for ssh-keyscan.
Mass move of bugs RESOLVED->CLOSED following the release of openssh-5.5p1