Use alien indexing method on all OSes

Emacs on Windows is slower than in Linux, and the fact that the default
indexing method on Windows is native makes it unusable in Windows except
for small projects (which still have a brief delay).

In reality, alien indexing works well enough in Windows with external
programs, and fast. For large projects with 30-40k files, retrieving
file list on Windows is only a few seconds slower.