Bug#855078: giac: FTBFS: [algo.pdf] Error 139 (Segmentation fault)

[Edmund Grimley Evans]

I was able to build giac 1.2.3.25+dfsg1-3 on arm64 with this "patch":

perl -i -pe 's/^#ifdef __x86_64__$/#if 1/;' src/gen.h
perl -i -pe 's/^#ifndef __x86_64__$/#if 0/;' src/first.h

Obviously that change would break it on 32-bit architectures. A proper
fix might be to use something like ~(uintptr_t)3 in gen.h, avoiding
the preprocessor, and in first.h something like:

#include <stdint.h>
#if UINTPTR_MAX < 0xffffffffffffffff