Out of tree builds were broken in commit 46b736f42e1165 since we didn't
explicitly use $(srcdir) to find the input files for glib-mkenums and
glib-genmarshal.
The rules to create signal marshallers and enumeration GTypes are
usually copied and pasted all over different projects, though they
are pretty generic and, given a little bit of parametrization, can
be put in separate Makefile.am files and included whenever needed.