Explicitly substitute abs_builddir, abs_srcdir

automake < 1.10 didn’t do this by default, resulting in failures of
‘make check’ and the ‘./barnowl’ wrapper script.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Distribute files in doc/ individually

‘make distcheck’ in automake < 1.11 throws permission errors when some
files in doc/ are distributed twice:
  http://sourceware.org/cgi-bin/gnatsweb.pl?cmd=view&pr=533

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
configure: Support --disable-maintainer-mode

automake < 1.8 doesn’t distribute m4/*.m4 even though it generates a
rule for aclocal.m4 that depends on them.  Supporting
--disable-maintainer-mode allows ‘make distcheck’ to disable this
rule.  Whatever.  It might be handy to have around anyway.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>