Fix AltiVec bool keyword conflict with -Dbool=_Bool

GCC with -maltivec and -std=c11 redefines 'bool' as '__vector __bool int'
in some contexts, breaking all uses of scalar bool in the HACL* BLAKE2
SIMD128 code. The -Dbool=_Bool flag forces the C99 scalar boolean type,
which is what HACL* intends.

This is a known GCC/AltiVec interaction: the AltiVec extension adds
'bool' as a keyword for vector boolean types, conflicting with C99/C11
stdbool.h. The -Dbool=_Bool workaround is used by several other projects
including FFmpeg and OpenSSL.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Scottcjn

configure

@@ -8133,7 +8133,7 @@ then
     dnl x86 SSE not available; check for PowerPC AltiVec/VSX (Power8+).
     dnl HACL* libintvector.h has a complete vec128 implementation for __powerpc64__.
     AX_CHECK_COMPILE_FLAG([-maltivec -mvsx],[
-      [LIBHACL_SIMD128_FLAGS="-maltivec -mvsx -flax-vector-conversions -Wno-return-type"]
+      [LIBHACL_SIMD128_FLAGS="-maltivec -mvsx -flax-vector-conversions -Wno-return-type -Dbool=_Bool"]
 
       AC_DEFINE([_Py_HACL_CAN_COMPILE_VEC128], [1], [
         HACL* library can compile SIMD128 implementations])