Fix 'source file in a subdirectory' Automake warnings

* configure.ac (AM_INIT_AUTOMAKE): Enable subdir-objects (to avoid
future incompatibilities as "cord" and "tests" source files are in
subdirectories).
Fix (enable) Cygwin-64 build

* include/gc.h (GC_DATASTART, GC_DATAEND): Define for Cygwin-64.
* include/private/gcconfig.h (X86_64): Likewise.
* include/private/gcconfig.h (OS_TYPE, DATASTART, DATAEND, STACK_GRAN,
NEED_FIND_LIMIT, USE_MMAP_ANON): Define for Cygwin-64 (code copied
from Cygwin-32 block).
* os_dep.c (GC_get_stack_base): Get the stack base for Cygwin-64
(implementation based on NtCurrentTeb call).
Update AUTHORS file
Remove barrett_diagram file duplicated by tree.html

* Makefile.direct (DOC_FILES): Remove doc/barrett_diagram entry.
* Makefile.dj (OTHER_FILES): Likewise.
* doc/doc.am (dist_pkgdata_DATA): Likewise.
* doc/barrett_diagram: Remove file (as the content is duplicated in
doc/tree.html).
Remove non-existing DISCARD_WORDS from GC data structure ASCII diagram

* doc/tree.html (picture): Remove DISCARD_WORDS (since always zero
and already removed from the code in gc-6.0alpha6).