Define CLANG/GNUC_PREREQ macros to check gcc/clang minimum version

Define CLANG/GNUC_PREREQ macros to check gcc/clang minimum version
(code refactoring)

* cord/cordxtra.c (CORD_ATTR_UNUSED): Use GC_GNUC_PREREQ() instead of
direct use of __GNUC[_MINOR]__.
* gc_cpp.cc (GC_NEW_DELETE_NEED_THROW): Likewise.
* include/gc_config_macros.h (GC_API, GC_ATTR_MALLOC, GC_ATTR_ALLOC_SIZE,
GC_ATTR_NONNULL, GC_ATTR_DEPRECATED, GC_RETURN_ADDR,
GC_RETURN_ADDR_PARENT, GC_PTHREAD_EXIT_ATTRIBUTE): Likewise.
* include/gc_cpp.h (GC_NO_OPERATOR_NEW_ARRAY): Likewise.
* include/gc_inline.h (GC_EXPECT, GC_PREFETCH_FOR_WRITE): Likewise.
* include/new_gc_alloc.h: Likewise.
* include/private/gc_priv.h (GC_INNER, GC_ATTR_UNUSED, EXPECT, GC_INLINE,
GC_ATTR_NOINLINE, GC_API_OSCALL, GC_ATTR_FORMAT_PRINTF,
NONNULL_ARG_NOT_NULL): Likewise.
* include/private/gc_priv.h [CANCEL_SAFE] (GC_cancel_disable_count):
Likewise.
* include/private/gcconfig.h (HAVE_BUILTIN_UNWIND_INIT, PREFETCH,
GC_PREFETCH_FOR_WRITE): Likewise.
* include/private/gcconfig.h [I386 && LINUX && __ELF__]
(GC_NO_SIGSETJMP): Likewise.
* include/private/gcconfig.h [X86_64 && MSWIN32] (MPROTECT_VDB):
Likewise.
* include/private/thread_local_alloc.h [CYGWIN32] (USE_PTHREAD_SPECIFIC):
Likewise.
* include/private/thread_local_alloc.h [LINUX && !ARM32 && !AVR32
|| PLATFORM_ANDROID && !__clang__] (USE_COMPILER_TLS): Likewise.
* mark.c [MSWIN32 || MSWINCE] (GC_mark_some): Likewise.
* mark_rts.c (GC_approx_sp): Likewise.
* tests/test_cpp.cc (ATTR_UNUSED): Likewise.
* tools/setjmp_t.c (nested_sp): Likewise.
* include/gc_config_macros.h (GC_GNUC_PREREQ): New macro (not for
public use).
* include/private/gc_priv.h (GC_CLANG_PREREQ): New private macro.
* include/private/gcconfig.h (HAVE_BUILTIN_UNWIND_INIT): Refine comment.
* include/private/thread_local_alloc.h [LINUX && !ARM32 && !AVR32
|| PLATFORM_ANDROID && !__clang__] (USE_COMPILER_TLS): Use
GC_CLANG_PREREQ() instead of direct use of __clang_major__ and
__clang_minor__.
* mark.c [MSWIN32 || MSWINCE] (GC_mark_some): Likewise.