configure.ac: Fix --with-xorg-conf-dir default value

This fixes the exact same issue seen here at libinput:

https://cgit.freedesktop.org/xorg/driver/xf86-input-libinput/commit/?id=0cfe9ec6c23e73507fd0797bae24c5ed6fcce033

Author: merge

configure.ac

@@ -77,9 +77,9 @@ AC_SUBST(inputdir)
 
 AC_ARG_WITH(xorg-conf-dir,
             AC_HELP_STRING([--with-xorg-conf-dir=DIR],
-                           [Default xorg.conf.d directory [[default=$prefix/share/X11/xorg.conf.d/]]]),
+                           [Default xorg.conf.d directory [[default=${prefix}/share/X11/xorg.conf.d/]]]),
             [xorgconfdir="$withval"],
-            [xorgconfdir="$prefix/share/X11/xorg.conf.d"])
+            [xorgconfdir='${prefix}/share/X11/xorg.conf.d'])
 AC_SUBST(xorgconfdir)
 
 # Checks for extensions