Disable automake portability warnings

automake 1.13 enables portability warnings by default now, causing errors with
gtk-doc 1.19:

  automake: warnings are treated as errors
  docs/gtk-doc.make:77: warning: GTK_DOC_V_SETUP_$(V: non-POSIX recursive variable expansion
  docs/reference/Makefile.am:90:   'docs/gtk-doc.make' included from here
  docs/gtk-doc.make:78: warning: GTK_DOC_V_SETUP_$(AM_DEFAULT_VERBOSITY: non-POSIX recursive variable expansion

and so on. Disable the portability warnings so that autogen.sh works with these
versions.