bool and C23

[Paul Eggert]

I installed the attached patch into Gnulib, reflecting a patch I 
recently installed into Autoconf 
<https://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=6dcecb780a69bd208088d666b299e92aa7ae7e80>.

I think we need a new Autoconf macro that obsoletes AC_HEADER_STDBOOL 
and AC_CHECK_HEADER_STDBOOL. This new macro should arrange for 'bool', 
'true' and 'false' to work, without programs having to include 
stdbool.h. For C23 and C++ the new macro should do nothing. For C99 
through C17 it should include <stdbool.h> in config.h. For pre-C99 it 
should #define bool, true, and false in config.h.

Eventually Gnulib and other code can stop using AC_*HEADER_CHECKBOOL and 
stop including <stdbool.h>.

I'm thinking of calling the new macro AC_C_BOOL.  Gnulib can have a new 
c-bool module that implements AC_C_BOOL for platforms running older 
Autoconf.

Comments?

0001-stdbool-port-to-C23.patch
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