EXTRA_HEADERS versus make dist [test]

[Kevin Ryde]

In 1.4f files given as

        EXTRA_HEADERS = foo.h

don't get into a "make dist".  Is this meant to work?  It did in
recent cvs versions but I guess noinst_HEADERS might be the right way
to do it.  A bit of a test below if it is meant to work.

EXTRA_HEADERS also don't seem to get into $(HEADERS) for TAGS etc, but
that might have been so for a while, and perhaps is intentional.

        * tests/distheaders.test: New file, derived from distname.test.
        * tests/Makefile.am (TESTS): Add it.

distheaders.test