cmake, autotools: Add find package config support for cmake clients

cmake, autotools: Add find package config support for cmake clients

With this support cmake and autotools generates cmake equivalent of
pkgconfig files on configure time named DBus1Config*.cmake. These
files are installed into the related directory where cmake expects
find_package related config files.

For instructions how to use this feature with clients see readme.cmake.

With previous DBus versions each cmake client using DBus as dependency
needed a related FindDBus*.cmake in its source distribution or in
the cmake binary packages. With the 'config' find package style support
provided by this patch this requirement has been removed.

The generated config file uses pkgconfig on unix or autotools to
fetch package build flags, which is the prefered way. On Windows
we do not want to require CMake users to have pkg-config installed
so it uses cmake buildin target export support for exporting all
targets into DBus1ConfigTargets*.cmake.

[smcv: make sure variable substitution works in Autotools too]

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=99721
Reviewed-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>
Signed-off-by: Simon McVittie <smcv@collabora.com>