patch: fix ETCSCREENRC

Hi,

The location of screenrc should be defined by sysconfdir, not prefix.

Most Linux distributions install screen in prefix /usr and have the
system configuration in /etc. Without this patch screenrc will be
located in /usr/etc/screenrc.

Please apply the following patch:

--- Makefile.in.orig	2008-11-15 20:47:44.000000000 +0100
+++ Makefile.in	2008-11-15 20:48:21.000000000 +0100
@@ -15,6 +15,7 @@
 exec_prefix = @exec_prefix@
 datarootdir = @datarootdir@
 datadir = @datadir@
+sysconfdir = @sysconfdir@
 
 # don't forget to change mandir and infodir in doc/Makefile.
 bindir  = $(exec_prefix)/bin
@@ -25,7 +25,7 @@
 GIT_REV = "`git describe --always 2>/dev/null`"
 DEFS = @DEFS@ -DGIT_REV=\"$(GIT_REV)\"
 
-ETCSCREENRC = $(prefix)/etc/screenrc
+ETCSCREENRC = $(sysconfdir)/screenrc
 SCREENENCODINGS = $(datadir)/screen/utf8encodings
 
 CC = @CC@