Allow to change config file (sub)directory

This allows the `xrdp` part of the path `/etc/xrdp` where config files
are placed to be customizable. This change is useful when trying the
stable version and the devel version alternately.

mc/Makefile.am

@@ -1,5 +1,5 @@
 AM_CPPFLAGS = \
-  -DXRDP_CFG_PATH=\"${sysconfdir}/xrdp\" \
+  -DXRDP_CFG_PATH=\"${sysconfdir}/${sysconfsubdir}\" \
   -DXRDP_SBIN_PATH=\"${sbindir}\" \
   -DXRDP_SHARE_PATH=\"${datadir}/xrdp\" \
   -DXRDP_PID_PATH=\"${localstatedir}/run\" \