entity-manager: Search sysconfdir for configurations

Make it easier to iteratively develop configurations in-place with a
read-only /usr.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Id0853b579a61394cb2b6ef0a3858ba01e5cb2893
diff --git a/meson.build b/meson.build
index 16d86c5..bb5da8c 100644
--- a/meson.build
+++ b/meson.build
@@ -60,6 +60,11 @@
     get_option('datadir'),
     meson.project_name(),
 )
+sysconfdir = join_paths(
+    get_option('prefix'),
+    get_option('sysconfdir'),
+    meson.project_name(),
+)
 threads = dependency('threads')
 if cpp.has_header('valijson/validator.hpp')
     valijson = declare_dependency()