Add support for --enable-exampledir to specify where to place example
files, defaulting to sysconfdir (matching previous behavior).

This is needed to support (cleanly) NetBSD pkgsrc, which requires that
example config files go in $(prefix)/share/examples/pkgname, rather
than in $(prefix)/etc/pkgname.
diff --git a/zebra/Makefile.am b/zebra/Makefile.am
index d449fe1..6f8e954 100644
--- a/zebra/Makefile.am
+++ b/zebra/Makefile.am
@@ -43,4 +43,6 @@
 #	$(CC) -g -o client client_main.o ../lib/libzebra.a $(LIBS) $(LIB_IPV6)
 
 quaggaconfdir = $(sysconfdir)
-dist_quaggaconf_DATA = zebra.conf.sample
+
+examplesdir = $(exampledir)
+dist_examples_DATA = zebra.conf.sample