build: allow configure and build in a separate directory

Some .h files in lib/ are autogenerated. The search path should
include the build directory and the source directory. They usually
match but sometimes, they may be different. For example:

 $ mkdir build
 $ cd build
 $ ../configure
 $ make
diff --git a/watchquagga/Makefile.am b/watchquagga/Makefile.am
index a49f62e..badaa5b 100644
--- a/watchquagga/Makefile.am
+++ b/watchquagga/Makefile.am
@@ -1,6 +1,6 @@
 ## Process this file with Automake to create Makefile.in
 
-INCLUDES = @INCLUDES@ -I$(top_srcdir) -I$(top_srcdir)/lib
+INCLUDES = @INCLUDES@ -I$(top_srcdir) -I$(top_srcdir)/lib -I$(top_builddir)/lib
 DEFS = @DEFS@ -DSTATEDIR=\"$(localstatedir)/\"
 
 AM_CFLAGS = $(PICFLAGS)