FreeTDM: Escape $(srcdir) in configure.ac variable

Silencing a "configure:xxxx: srcdir: command not found" error message.
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

libs/freetdm/configure.ac

@@ -54,7 +54,7 @@ fi
 
 AC_SUBST([confdir])
 
-DEFAULT_INCLUDES="-I. -I./src/include -I$(srcdir)"
+DEFAULT_INCLUDES="-I. -I./src/include -I\$(srcdir)"
 AC_SUBST([DEFAULT_INCLUDES])
 
 # Where to install the modules