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/vtysh/extract.pl.in b/vtysh/extract.pl.in
index 61b2d2a..99224f8 100755
--- a/vtysh/extract.pl.in
+++ b/vtysh/extract.pl.in
@@ -63,7 +63,7 @@
 foreach (@ARGV) {
     $file = $_;
 
-    open (FH, "cpp -DHAVE_CONFIG_H -DVTYSH_EXTRACT_PL -DHAVE_IPV6 -I@top_builddir@ -I@srcdir@/ -I@srcdir@/.. -I@top_srcdir@/lib -I@top_srcdir@/isisd/topology @SNMP_INCLUDES@ @CPPFLAGS@ $file |");
+    open (FH, "cpp -DHAVE_CONFIG_H -DVTYSH_EXTRACT_PL -DHAVE_IPV6 -I@top_builddir@ -I@srcdir@/ -I@srcdir@/.. -I@top_srcdir@/lib -I@top_builddir@/lib -I@top_srcdir@/isisd/topology @SNMP_INCLUDES@ @CPPFLAGS@ $file |");
     local $/; undef $/;
     $line = <FH>;
     close (FH);