meson.build: derive dbuspolicydir from libdbus datadir

All other dbus directory locations are derived from libdbus pkg-config
variables.
The dbuspolicydir instead only used the 'datadir' which has a fixed
default and might thus not be valid on some distributions.

If the value cannot be obtained from pkg-config, the fallback remains
the same as before.

Fixes rauc#1479

Signed-off-by: Enrico Joerns <[email protected]>

meson.build

@@ -227,9 +227,11 @@ if dbussystemservicedir == ''
   endif
 endif
 
+dbusdatadir = dbusdep.get_variable(pkgconfig: 'datadir', default_value: datadir) / 'dbus-1'
+
 dbuspolicydir = get_option('dbuspolicydir')
 if dbuspolicydir == ''
-  dbuspolicydir = datadir / 'dbus-1' / 'system.d'
+  dbuspolicydir = dbusdatadir / 'system.d'
 endif
 
 dbusinterfacesdir = get_option('dbusinterfacesdir')