MingW32 builds: generate NSIS installers for Windows

MingW32 builds: generate NSIS installers for Windows

nsiswrapper is a rather dodgy Perl script (looks like it hasn't actually
been updated since ~2009) with one indispensable function: it can
automatically find required DLLs and bundle them into the installer.

1. The `PATH` variable must be set to include DLL locations (why `PATH`?️)
2. It mixes up its stdout and stderr, so `--verbose` can't be used without
   `--run`. (Argh…)
3. It doesn't try to normalize the paths of the bundled files, so
   `./x` and `$PWD/x` result in different directory structures.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>