vars.am: Fix include path tweaking.

vars.am: Fix include path tweaking.

For some reason, the include directories were set in CPPFLAGS rather
than AM_CPPFLAGS, which is just a mistake.

Also, astonishingly, the makefile botched adding $(top_builddir)/config
to the include path.  This obviously never worked.  Fortunately,
Automake magically included the correct path by itself, so just remove
the embarrassing mistake and pretend it never happened.