Use @docdir@ macro instead of semi-hard coded directory value.

diff --git a/doc/Makefile.in b/doc/Makefile.in
index fd5bae8..bd5506e 100644
--- a/doc/Makefile.in
+++ b/doc/Makefile.in
@@ -199,7 +199,7 @@ build_vendor = @build_vendor@
 builddir = @builddir@
 datadir = @datadir@
 datarootdir = @datarootdir@
-docdir = $(datadir)/xnee
+docdir = @docdir@
 dvidir = @dvidir@
 exec_prefix = @exec_prefix@
 gtk_CFLAGS = @gtk_CFLAGS@