use noinst_HEADERS for all internal headers

Not really sure why these have been in libdiscid_la_SOURCES before
and I hope this doesn't break somehow.
LIB-26: run test_disc, but skip it without disc

Autotools supports a return value of 77 to skip the test
so we do that.
Cmake doesn't support testind directly anyways so we have
to catch the return type 77 manually.

return values other than 0 and 77 should lead to errors of course.