Makefile.autotools.in: Add extra POST_HOOK targets

It is not possible currently in Makefile.autotools.in packages 
to do something extra after an extract or configure. 
This may be useful for example 
extract libxml2-2.6.32 
POST_EXTRACT = remove some .rej files that are in offical release
patch (not fail because of remaining .rej files)
configure

I think this is useful improvement 
(necessary for Makefile.autotools.in packages)

Daniel Laird