Currently, the way src/sct.h is generated forbids out-of-tree builds.

Fix this by giving the util/gen_public_header.pl script full paths
to the source headers, i.e. prefix them by $(srcdir).

For the tests directory, add -I $(top_builddir)/src to its CPPFLAGS in
order to make the tests compilations find the sct.h places in the build
directory now.