skip using gcc brace groups for STMT_START/END

[tonycoz]

This warns (and warns a lot) on clang, and since these are documented
to only work to make a single statement, so there's little value to
allowing them to work in an expression.

An alternative would be to disable GCC brace groups on clang, but
these are used extensively in DEBUGGING builds to add extra checks
in sv.h.

Fixes #18780

[khwilliamson]

This breaks Devel::PPPort at perl 5.13.0, and needs to be reverted or fixed before the next monthly development release.

The errors are:

'stderr' => [
'In file included from RealPPPort.xs:146:0:
',
'RealPPPort.xs: In function ‘XS_Devel__PPPort_eval_sv’:
',
'ppport.h:5867:25: error: expected expression before ‘do’
',
' # define STMT_START do
',
' ^
',
'ppport.h:8160:5: note: in expansion of macro ‘STMT_START’
',
' STMT_START { \
',
' ^
',
'ppport.h:8508:121: note: in expansion of macro ‘croak_sv’
',
' # define D_PPP_CROAK_IF_ERROR(cond) ({ SV _errsv; ((cond) && (_errsv = ERRSV) && (SvROK(_errsv) || SvTRUE(_errsv)) && (croak_sv(_errsv), 1)); })
',
' ^
',
'ppport.h:8533:113: note: in expansion of macro ‘D_PPP_CROAK_IF_ERROR’
',
' # define eval_sv(sv, flags) ({ I32 _flags = (flags); I32 ret = Perl_eval_sv(aTHX sv, (_flags & ~G_RETHROW)); D_PPP_CROAK_IF_ERROR(_flags & G_RETHROW); _ret; })
',
' ^
',
'RealPPPort.xs:802:21: note: in expansion of macro ‘eval_sv’
',
' i = eval_sv(sv, flags);
',
' ^
',
'In file included from /media/Tux/perls-t/lib/5.13.0/x86_64-linux-thread-multi-ld/CORE/perl.h:4934:0,
',
' from RealPPPort.xs:31:
',
'RealPPPort.xs: In function ‘XS_Devel__PPPort_eval_pv’:
',
'ppport.h:5867:25: error: expected expression before ‘do’
',
' # define STMT_START do
',
' ^
',
'/media/Tux/perls-t/lib/5.13.0/x86_64-linux-thread-multi-ld/CORE/pp.h:291:28: note: in definition of macro ‘PUSHs’
',
' #define PUSHs(s) (++sp = (s))
',
' ^
',
'ppport.h:8160:5: note: in expansion of macro ‘STMT_START’
',
' STMT_START { \
',
' ^
',
'ppport.h:8508:121: note: in expansion of macro ‘croak_sv’
',
' # define D_PPP_CROAK_IF_ERROR(cond) ({ SV *_errsv; ((cond) && (_errsv = ERRSV) && (SvROK(_errsv) || SvTRUE(_errsv)) && (croak_sv(_errsv), 1)); })
',
' ^
',
'ppport.h:8544:78: note: in expansion of macro ‘D_PPP_CROAK_IF_ERROR’
',
' # define eval_pv(p, croak_on_error) ({ SV *sv = Perl_eval_pv(aTHX p, 0); D_PPP_CROAK_IF_ERROR(croak_on_error); _sv; })
',
' ^
',
'RealPPPort.xs:814:23: note: in expansion of macro ‘eval_pv’
',
' PUSHs(eval_pv(p, croak_on_error));
',
' ^
',
'make: *** [RealPPPort.o] Error 1
'

[khwilliamson]

I did some more digging, and the reason is related to this:
commit 1400a53
Author: Nicholas Clark [email protected]
Date: Wed Sep 11 11:54:42 2013 +0200

 Eliminate the only use of VOIDFLAGS, as part of STMT_START in perl.h
 
 STMT_START has used VOIDFLAGS as part of its conditional compilation since it
 was added by commit 728e280303ae3d3b (March 1996). The code originally read:
 
    /* Now which other defined()s do we need here ??? */
 
 Since then it has been amended to avoid entering that definition for GCC
 on Solaris. Given that all current Solaris compilers are C89 conformant,
 VOIDFLAGS will always be true (actually 15), so the test is redundant.
 Even back in 1996, it's possible that VOIDFLAGS was always non-zero on
 SunOS, rendering the test obsolete from the start.
 
 Spotted by Brian Fraser, and extracted from a larger patch of his.