configure: revert to using GNU defaults, update summary

- Changing the GNU defaults is messy.  The best way is to leave them
  alone and let the user set their install locations themselves.
- Show --exec-prefix in config Summary, that's where the binaries go

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

contrib/void/build.sh

@@ -14,7 +14,7 @@ fi
 # The plugins are optional, but you may need D-Bus and X11 if you want
 # to run X-Window, the other configure flags are however required.
 PKG_CONFIG_LIBDIR=/usr/lib/pkgconfig:/usr/local/lib/pkgconfig ./configure	\
-		                               --enable-progress		\
+		 --prefix=/usr --exec-prefix=  --enable-progress		\
                  --enable-dbus-plugin          --enable-x11-common-plugin	\
 		 --enable-alsa-utils-plugin    --enable-inetd-echo-plugin	\
 		 --enable-inetd-chargen-plugin --enable-inetd-daytime-plugin	\