Fix test_cpp (ensure the collector recognizes pointers to interiors)

* tests/test_cpp.cc (main): Call GC_set_all_interior_pointers(1)
before GC_INIT to ensure that the collector considers pointers to
object interiors as valid ones (such a pointer could emerge as
a result of a type cast to subclass in case of multiple inheritance);
add comment.
test_cpp: Fix WinMain to prevent SEGV if zero arguments passed (MinGW)

* tests/test_cpp.cc (WinMain): Prevent SEGV in strtok() by checking
"cmd" WinMain argument for NULL (in that case "argc" local variable is
set to 0).
Fix thread-related tests for pthreads-w32

* tests/initsecondarythread.c: Include "private/config.h" if
HAVE_CONFIG_H (mostly to have GC_WIN32_PTHREADS defined for
pthreads-w32 target).
* tests/thread_leak_test.c: Likewise.
* tests/threadkey_test.c: Likewise.
Update ChangeLog
Fix stop_info.stack_ptr assignment in GC_suspend_all for OpenBSD
(commit '7ff92dc' from master branch)

* os_dep.c (GC_get_stack_base): Abort if pthread_stackseg_np fails
(if GC_OPENBSD_THREADS).
* pthread_stop_world.c (GC_suspend_all): Get correct stack_ptr by
calling pthread_stackseg_np (subtracting ss_size from ss_sp) instead
of retrieving it from OpenBSD pthread implementation-dependent context
(if GC_OPENBSD_THREADS); remove comment.
Update ChangeLog
Fix GC_call_with_stack_base to prevent its tail-call optimization

* misc.c (GC_call_with_stack_base): Call GC_noop1 after fn()
invocation to prevent a tail-call optimization.
Fix all address-of-dummy operations by using GC_approx_sp() instead
(a joint commit of commits 'd6acbd' and '57b94a3' in master branch)

* alloc.c (min_bytes_allocd, GC_stopped_mark): Use GC_approx_sp()
instead of "&dummy"; remove 'dummy' local variable.
* dyn_load.c (GC_cond_add_roots): Likewise.
* misc.c (GC_init): Likewise.
* os_dep.c (GC_get_stack_base, GC_get_main_stack_base): Likewise.
* pthread_stop_world.c (GC_suspend_handler_inner,
nacl_pre_syscall_hook, __nacl_suspend_thread_if_needed): Likewise.
* pthread_support.c (GC_thr_init): Likewise.
* ptr_chck.c (GC_on_stack): Likewise.
* win32_threads.c (GC_push_stack_for): Likewise.
* extra/setjmp_t.c (main): Define volatile 'sp' local variable, store
its address to it and use it instead of "&dummy"; remove 'dummy' local
variable.
* mach_dep.c (GC_with_callee_saves_pushed): Use volatile for 'dummy'
local variable.
* misc.c (GC_clear_stack_inner): Store address of volatile 'dummy'
local array (i.e. 'sp' value) to its first element (and use it in the
comparison of addresses) to prevent any harmful optimizations as C
officially disallows comparisons of pointers to different objects
(e.g., some Mac OS X clang releases might turn a conditional
expression that uses 'dummy' address into a constant); update comment.
* misc.c (GC_call_with_stack_base): Use "&base" instead of "&dummy"
(it is safe to use address of base here); remove dummy variable.
Fix GC_clear_stack_inner by using GC_approx_sp

* misc.c (GC_clear_stack_inner): Use GC_approx_sp() instead of
"dummy[0]" set to "&dummy" value (that worked as expected only for
STACK_GROWS_DOWN case).
Update ChangeLog
Update (regenerate) config.guess/sub, libtool.m4; update ChangeLog

* config.guess: Regenerate (by autoreconf -vif using autoconf-2.68,
automake-1.11.1 and libtool-2.4).
* config.sub: Likewise.
* m4/libtool.m4: Likewise.
Merge branch 'gc-7_2-hotfix-3' into release-7_2
[7.2d]

Change version in README to 7.2d; update ChangeLog