CMake: Throw a clear error when config.h in top-level source directory

CMake: Throw a clear error when config.h in top-level source directory

This causes weird, difficult to debug compilation errors. Usually
caused by trying to run CMake against a source that had an
in-tree autoconfig build.

Note that you're able to do out-of-tree autoconfig builds and
they can obviously mix with out-of-tree cmake builds.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20230703153925.48990-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26809.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>