Running

    ./pre-inst-env guix build --target=x86_64-w64-mingw32 hello

on core-updates fails with

--8<---------------cut here---------------start------------->8---
../../../../gcc-7.4.0/libstdc++-v3/libsupc++/new_opa.cc: In function ‘void* __gnu_cxx::aligned_alloc(std::size_t, std::size_t)’:
../../../../gcc-7.4.0/libstdc++-v3/libsupc++/new_opa.cc:78:10: error: ‘memalign’ was not declared in this scope
   return memalign (al, sz);
          ^~~~~~~~
../../../../gcc-7.4.0/libstdc++-v3/libsupc++/new_opa.cc:78:10: note: suggested alternative: ‘max_align_t’
   return memalign (al, sz);
          ^~~~~~~~
          max_align_t
--8<---------------cut here---------------end--------------->8---

The attached patch fixes this and produces a working cross-built hello.

The weird thing is that I looked at Debian's mingw cross compilers and
its mingw-w64 package and cannot seem to find anything that resembles
this build problem or a fix.

Greetings,
janneke