The include/ dir is somewhat redundant and makes it just a bit harder to
handle the -I directives from out side of automake; without it the
default $(top_buildir) just works.
Here's also some further justifications I found:
http://smcv.pseudorandom.co.uk/2008/09/pc-uninstalled/
Signed-off-by: Ran Benita <ran234@gmail.com>