Add subproject directory & wrapper files

This commit enables the isolated build of this repository with meson.
This contribution was made possible by Patrick's [blog post][1].

Building standalone with `meson setup builddir && ninja -C builddir`
worked up to the point when `phosphor-logging` raised an
[error][2]. So I cannot state that it is fully tested, but setting up
the `builddir` correctly should've been half the way.

[1]: http://www.stwcx.xyz/blog/2021/04/18/meson-subprojects.html
[2]: https://github.com/openbmc/phosphor-logging/issues/22

Signed-off-by: Patrik Tesarik <patrik.tesarik@rub.de>
Change-Id: I8ce6c9292edbb6d72414edd8794c337f30bc0734
diff --git a/meson.build b/meson.build
index 43e4221..ed8e7d1 100644
--- a/meson.build
+++ b/meson.build
@@ -27,9 +27,9 @@
 
 deps = [dependency('boost'),
         dependency('boost', modules : ['coroutine']),
-        dependency('phosphor-dbus-interfaces'),
-        dependency('phosphor-logging'),
-        dependency('sdbusplus'),
+        dependency('phosphor-dbus-interfaces', fallback: ['phosphor-dbus-interfaces', 'phosphor_dbus_interfaces_dep']),
+        dependency('phosphor-logging', fallback: ['phosphor-logging', 'phosphor_logging_dep']),
+        dependency('sdbusplus', fallback: ['sdbusplus', 'sdbusplus_dep']),
         dependency('systemd'),
 ]