Add macro for checking apr/apu defines like APR_HAS_feature,

Add macro for checking apr/apu defines like APR_HAS_feature,
and for now use it to detect APR_HAS_XLATE and APU_HAVE_CRYPTO.

Add special logic for special APR_HAS_LDAP.

Display summary of feature detection.

mod_session_crypto can now build, so do so if the prereq is present.
(By default it won't be activated.)

Sketch in more support for Lua, mod_charset_lite, mod_ldap, and
mod_authnz_ldap, though  they should be expected to fail until
somebody actually tries it and fixes the problem.

The documented ab issue was my own problem with a mismatch between
my compiler and the OpenSSL compiler, resolved by building OpenSSL
myself like everybody else.

Document another of the possibly zillions of bugs/limitations.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1520783 13f79535-47bb-0310-9956-ffa450edef68