GitLab

This is required to run the compiler.

It is now necessary to clean-up locally some MinGW bits included in the Octave
package, since they interfere with our cross-compiler.

Some packages are downgraded, since this is what happened in MSYS2 package
manager (presumably because of bugs/stability issues).

Extracting the NSIS installer no longer works for the 64-bit version. Rather
use the 7z archives directly.

Add a potential suffix to the name of the 7z archive downloaded, to allow
selecting a specific build (for 4.4.0 under w64, the first build and the second
one, suffixed "_1", are not ABI-compatible).

Remove the -std=c++11 flag in mkoctfile. It breaks the check for C++14.

Use official GNU mirror CDN (using HTTPS). By the way, also use the CDN for
downloading Octave Windows binaries.

Taget all has to call target dll (downloads libquadmath dlls).