[configure.in] Clean up library testing.

[configure.in] Clean up library testing.
Add optional PATH to --with-pcre, --with-zlib, --with-tcp-wrappers
based on patch by albert chin (china@thewrittenword.com)

Re-arange AC_CHECK_HEADERS and AC_CHECK_FUNCS for eaiser reading
of patches to configure.in

Replace obsolete AC_STRUCT_ST_BLKSIZE with AC_CHECK_MEMBERS

Add test for broken dirname() on Solaris 2.5.1 by
Dan Astoorian <djast@cs.toronto.edu>

[acconfig.h aclocal.m4 defines.h configure.in]
Better socklen_t patch by albert chin (china@thewrittenword.com)

[scp.c]
Replace obsolete HAVE_ST_BLKSIZE with HAVE_STRUCT_STAT_ST_BLKSIZE

[Makefile.in] When running make in top level, always do make in openbsd-compat
patch by Dave Dykstra <dwd@bell-labs.com>