Define a macro NH_C to provide a shorter & simpler way to test for
which C standard the build is being carried out under (c99 or c23).
NH_C > 202300L Being compiled under C23 or greater
NH_C > 199900L Being compiled under C99 or greater
NH_C > 198900L Being compiled under C89 or greater,
or C std could not be determined.
While NetHack only requires c99, we've been taking advantage
of some c23 features (attributes), if they are available,
to allow the use of ATTRNORETURN/NORETURN and FALLTHROUGH on
compilers other than gcc.
Also add some comment documentation to tradstdc.h about NetHack's
use of c99.
The sys/unix/Makefile.top change overcomes a warning in the
Makefile-generated nhlua.h. That warning arises under some compilers
that rely on attribute [[noreturn]] ahead of a declaration
(NetHack macro ATTRNORETURN), rather than the trailing gcc
__attribute((noreturn)) (NetHack macro NORETURN). The sed command
is modified to include ATTRNORETURN at the start of the declaration
in addition to the NORETURN at the end of the declaration, in the
generated file. That's the same combination that's used for the
declaration of other functions that don't return.
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