The sct.h header is generated at build time and depends on the
configuration. Do not include it in 'make dist' by putting it into
the nodist_pkginclude_HEADERS variable.

Furthermore, remove the *_SOURCES dependencies on sct.h from the programs
in tests/ -- the Automake manual explicitly says that this doesn't work
(section 9.4.1 Built Sources Example) and they cause sct.h to be included
in the 'make dist' tarball.