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
Update ChangeLog file
Update ChangeLog file (related to 7.5.0)