configure: simpify dbus policy install dir

Using the package config to find where dbus-1 installs its config files
is needless indirection and adds a hard build-time dependency on the
dbus-1 package, which isn't necessary.  The `dbus-daemon` manpage
suggests that package-level configuration should be placed in
`/usr/share` and not `/etc`, so use `${datadir}` to place the config
files in that location.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I81ae7c2007f0a9fc7838cf287488f1c18c1a128f
diff --git a/configure.ac b/configure.ac
index c30e109..97e5321 100644
--- a/configure.ac
+++ b/configure.ac
@@ -85,7 +85,7 @@
 AC_ARG_WITH([dbuspolicydir],
         AS_HELP_STRING([--with-dbuspolicydir=DIR], [D-Bus policy directory]),
         [],
-        [with_dbuspolicydir=$($PKG_CONFIG --variable=sysconfdir dbus-1)/dbus-1/system.d])
+        [with_dbuspolicydir=${datadir}/dbus-1/system.d])
 AS_IF([test "x$with_dbuspolicydir" != "xno"],
       [AC_SUBST([dbuspolicydir], [$with_dbuspolicydir])]
 )