Enable ldap modules in 'all' and 'most' selections if ldap is compiled into apr-util

Bring part some parts of r1142938 which were removed by the big ldap revert r1150179. Original commit log: Use APR_ADDTO instead of APR_SETVAR or direct variable assignment. ...

Merge branch revert-ap-ldap: Revert ap_ldap integration due to veto by Graham Leggett Mailing list threads: http://mail-archives.apache.org/mod_mbox/httpd-dev/201106.mbox/%3C4192DC1D-C0B9-42BB-B614-C3A41290F18B@sharp.fm%3E http://mail-archives.apache.org/mod_mbox/httpd-dev/201107.mbox/%3C4E15E51E.4090700@rowe-clan.net%3E

Allow to specify module specific custom linker flags via the MOD_XXX_LDADD variables. Use APR_ADDTO instead of APR_SETVAR or direct variable assignment. This is especially useful when building mod_lua or mod_deflate against a lua resp. libz which are installed in non-standard locations. One can add "-R ..." to MOD_LUA_LDADD and MOD_DEFLATE_LDADD before configure to fix the RPATH/RUNPATH of those modules.

Several fixes for the ap_ldap build logic. This should fix compilation without --with-ldap.

Incorporate the ap_ldap incomplete API, as there is no interest or effort at APR to make this a complete abstraction, and it was voted 'off the island' with APR 2.0. This will allow httpd 2.3 to build against either apr-2.0 or apr+util 1.x.

Always make mod_*.h files available where they exist, modpath by modpath

* Make LDAP working with APR 2.0 again by using apr-config instead of apu-config which is gone in APR 2.0.

For *trunk/* development, presume at least apr-1.3 (now released) for seperation of ldap link options.

Add build files for the util_ldap module