make dist vs. conditional installdirs

[Ralf Corsepius]

Another issue with conditional installdirs:

Given this Makefile.am:
if HAVE_FOO
include_HEADERS = foo.h
lib_LIBRARIES = libfoo.a
libfoo_a_SOURCES = foo.c
endif

If condition HAVE_FOO is false, foo.h isn't distributed (isn't put into
the tarball during make dist), but foo.c is.

# tar tzvf autobug-0.0.tar.gz | grep foo
autobug-0.0/foo.c

I thought *_HEADERS were always distributed, similar to *_SOURCES.

Ralf