mingw guile.exe cross build patch series v9

[Jan Nieuwenhuizen]

Hi!

Finally I have the next iteration ready.  After adressing all comments on v8
and rebasing, the patches only built on core-updates.  I spent a some time to
find the problem with master and cherry-picking from core-updates before
giving up.

Now that core-updates has been merged the mingw cross patches build again.

As suggested by Ludovic, I have tested all individual patches with the
script below.

There are no additional changes except for adding gcc-cross-x86_64 package
descriptions to run these tests.

Greetings,
Jan

TEST
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#! /bin/bash -ex

make clean || true
./bootstrap
./configure --localstatedir=/var
make

./pre-inst-env guix build gcc-cross-sans-libc-x86_64-linux-gnu
./pre-inst-env guix build gcc-cross-x86_64-linux-gnu
./pre-inst-env guix build hello
./pre-inst-env guix build guile

./pre-inst-env guix build --target=mips64el-linux-gnuabi64 hello

if grep mingw gnu/packages/cross-base.scm; then
    ./pre-inst-env guix build --target=i686-w64-mingw32 hello
fi

if grep mingw gnu/packages/multiprecision.scm; then
    ./pre-inst-env guix build --target=i686-w64-mingw32 gmp
fi

if grep mingw gnu/packages/ncurses.scm; then
    ./pre-inst-env guix build --target=i686-w64-mingw32 ncurses
fi

if grep mingw gnu/packages/readline.scm; then
    ./pre-inst-env guix build --target=i686-w64-mingw32 readline
fi

if grep mingw gnu/packages/guile.scm; then
    ./pre-inst-env guix build --target=i686-w64-mingw32 guile
fi
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