Starting with rc6, there seem to be problem with compilation on openbsd 3.9, (possibly due to autoconf 2.60 switch mentioned in other thread ?). Either way, this is what configure reports at some point (further compilation fails of course):

checking for crypt... yes checking for dlopen... yes checking whether byte ordering is bigendian... no ./configure: echo $(top_builddir)/src/lib-storage/index/maildir/libstorage_maildir.a: not found ./configure: echo $(top_builddir)/src/lib-storage/index/mbox/libstorage_mbox.a: not found ./configure: echo $(top_builddir)/src/lib-storage/index/dbox/libstorage_dbox.a: not found checking zlib.h usability... yes checking zlib.h presence... yes checking for zlib.h... yes