build: fix a srcdir != builddir problem with gnetworking.h

gio's glib-mkenums call needs to get gnetworking.h out of $(builddir),
not $(srcdir). Fix/simplify it by using $(filter) on $^ and letting
make find everything.

Also add -Wno-portability to AM_INIT_AUTOMAKE in configure.ac, so that
it doesn't warn about this (or about the gmake-specific features we
were already using in gio/tests/)

https://bugzilla.gnome.org/show_bug.cgi?id=691866