Build failure due to pthread_rwlock_t

[barries]

This bit added recently to srclib/apr/include/arch/unix/locks.h:154

   #ifdef HAVE_PTHREAD_RWLOCK_INIT
       pthread_rwlock_t rwlock;
   #endif

blows up compiling on RH6.2 linux with stock build environment
(egcs-2.91.66, glib-devel v2.1.3, let me know if you need more info).

pthread_rwlock_t is only declared/defined if -D_XOPEN_SOURCE=500 (or
-D_GNU_SOURCE) but that causes other glitches.

- Barrie