s4: avoid overwritting the dceprc.h file from toplevel librpc

s4: avoid overwritting the dceprc.h file from toplevel librpc

Change back to what was the configuration of header_path before
177ddcb084e8f8608bf2012a7c58f7b1e56d2acd so that dcerpc.h is installed
in ${prefix}/include and not in ${prefix}/include/gen_ndr as the
toplevel librpc wants also to put a dcerpc.h file in this directory.

Add some documentation in order to avoid any new regression.

This change should fix problems for openchange team.

Autobuild-User: Matthieu Patou <mat@samba.org>
Autobuild-Date: Mon Feb 21 09:47:05 CET 2011 on sn-devel-104