If this new scheme of adding dependencies is supposed to be used more
widely we need to start making it easy to add them. So add a new
self-contained header that projects can simply include without the need
to link against libsystemd itself. This will allow them to generate
`.notes.dlopen` sections:
```
> readelf -p .note.dlopen ./l2md
String dump of section '.note.dlopen':
[ a] |@FDO
[ 10] [{"feature":"manifest-json","description":"Manifest-based change detection via gzip and JSON parsing","priority":"suggested","soname":["libz.so.1","libsystemd.so.0"]}]
[ c2] |@FDO
[ c8] [{"feature":"manifest-http","description":"HTTP transport for lore.kernel.org manifest fetch","priority":"suggested","soname":["libcurl.so.4"]}]
```
Luca Boccassi
cd1f2f2ff9