Require automake 1.7 or higher. Fixes problem reported by Gilad

Require automake 1.7 or higher.  Fixes problem reported by Gilad
Arnold where Gentoo Linux tries to be too smart and chooses automake
1.5, which doesn't set @PACKAGE@, leading to include files being in
$(prefix)/include rther then $(prefix)/include/quagga.
(Since our makefiles rely on @PACKAGE@, they should have required an
automake new enough to set it.)