build: Ensure that autogen exits with the right code

Since autogen.sh calls other commands that may fail, we should return
the exit code from those commands.

autogen.sh

@@ -6,9 +6,9 @@ test -z "$srcdir" && srcdir=.
 ORIGDIR=`pwd`
 cd "$srcdir"
 
-autoreconf -v --install || exit 1
+autoreconf -v --install || exit $?
 cd "$ORIGDIR" || exit $?
 
 if test -z "$NOCONFIGURE"; then
-    "$srcdir/configure" "$@"
+    exec "$srcdir/configure" "$@"
 fi