Makefile.am: allow manually installing config files

[vs49688]

Mostly to help with packaging in NixOS.

In NixOS, we set --prefix=/nix/store/xxxxxx/. However, we also have to use --without-configfiles, lest the Makefile attempts to install the config files and friends to /etc (which is a complete no-go).

Specifying DESTDIR=/nix/store/xxxxxx/ when using --prefix=/nix/store/xxxxxx/ causes the config files to be put in the right place, but the binaries will be at $DESTDIR/$PREFIX (in this case /nix/store/xxxxxx/nix/store/xxxxxx).

This PR adds a separate install-config-files Makefile target that installs the config files and dbus policies to DESTDIR=, regardless of whether --with{,out}-configfiles is specified. This target exists unconditionally.

To keep the existing make install behaviour, install-exec-hook is changed to depend on install-config-files only if INSTALL_CONFIG_FILES is set.

In effect, this allows us to use:

./configure --prefix=/nix/store/xxxxxx --without-config-files
make
make install

make DESTDIR=/nix/store/xxxxxx install-config-files

For context: NixOS/nixpkgs#276693

[vs49688]

Btw, my knowledge of autotools is antiquated, so I'm all-ears if there's a better way of doing this.

[mikebrady]

Thanks for this PR. Unfortunately, I'm away from my computers until the end of January, but I'll take a look at merging it then.

[mikebrady]

Hi there. Back at the coalface now. Would you be kind enough to set this up for the development branch instead of the master branch. We can merge in into there much more easily. It's probably easier just to withdraw the PR and make up a new one but based on the development branch...

[vs49688]

Hi there. Back at the coalface now. Would you be kind enough to set this up for the development branch instead of the master branch. We can merge in into there much more easily. It's probably easier just to withdraw the PR and make up a new one but based on the development branch...

Done, rebased and retargeted to development.

[mikebrady]

Many thanks for this -- let's try it out!