There were some Unix specific threading code added to apr_initialize() at some point, and WIN32 wasn't listed in the non-Unix OS's list. Shouldn't there be a UNIX define somewhere that can be used instead? (If need be, provided by a generic APR header; so as to get the list of "non-Unix" into a single location?)
Win32 uses apr/misc/win32/start.c
Perhaps you meant the cygwin port?
