c++: Predefine __STDCPP_THREADS__ in the compiler if thread model is not single ...

c++: Predefine __STDCPP_THREADS__ in the compiler if thread model is not single [PR63287]

The following patch predefines __STDCPP_THREADS__ macro to 1 if c++11 or
later and thread model (e.g. printed by gcc -v) is not single.
There are two targets not handled by this patch, those that define
THREAD_MODEL_SPEC.  In one case - QNX - it looks just like a mistake
to me, instead of setting thread_model=posix in config.gcc it uses
THREAD_MODEL_SPEC macro to set it unconditionally to posix.
The other is hpux10, which uses -threads option to decide if threads
are enabled or not, but that option isn't really passed to the compiler.
I think that is something that really should be solved in config/pa/
instead, e.g. in the config/xxx/xxx-c.c targets usually set their own
predefined macros and it could handle this, and either pass the option
also to the compiler, or say predefine __STDCPP_THREADS__ if _DCE_THREADS
macro is defined already (or -D_DCE_THREADS found on the command line),
or whatever else.

2020-11-13  Jakub Jelinek  <jakub@redhat.com>

	PR c++/63287
	* c-cppbuiltin.c: Include configargs.h.
	(c_cpp_builtins): For C++11 and later if THREAD_MODEL_SPEC is not
	defined, predefine __STDCPP_THREADS__ to 1 unless thread_model is
	"single".