cmake: symlink whole build dir not just .json file

cmake: symlink whole build dir not just .json file

It turned out that symlinking compile_commands.json from the top level
source dir has some issues:
  * file is not created on Windows and symlinking may cause an error
  * some IDEs create their own json and error out b/c a file exists

Since clangd also looks for the json in build/ directories by default,
we now symlink the whole build directory instead, not just the json file.

This approach requires for the existing build/ dir in the repo to
vanish. Luckily it only contains one automake include file, which is
moved to the top level source dir.

Lastly, make this an opt-in feature, so that the default configuration
of the buildsystem never causes a build failure because of this.

Change-Id: Ib1a5c788269949d8de95d1da2cb0c32a65bf13f2
Signed-off-by: Heiko Hund <heiko@ist.eigentlich.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20240117134929.5317-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28061.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>