Fix off-source build

off-source build is broken in 5ef0f343d219c9a9c3f7425251e06e97176411d2
by replacing INCLUDES with AM_CPPFLAGS

http://www.gnu.org/software/automake/manual/html_node/Program-Variables.html

> AM_CPPFLAGS is ignored in preference to a per-executable (or per-library) _CPPFLAGS variable
> if it is defined.