bug#19111: 25.0.50; 32 bits temacs.exe is linked with wrong image-base when built on 64 bit Windows host

[Óscar Fuentes]

Óscar Fuentes <ofv@wanadoo.es> writes:

> The problem here is that, without explicitly setting --host/--build,
> uname is not the right thing for detecting the host/build environment,
> because uname is a MSYS2 thing, when we are building with a MinGW
> toolset. So either we stop using uname for MinGW builds (use $MSYSTEM),
> or document that --host/--build are required.

There other places on `configure' that uses the contents of $canonical
for deciding if its creating an 32 or 64 bits MinGW build.