Use NTFS junctions to share/julia/base etc instead of Cygwin symlinks

[tkelman]

I would've committed this straight to master if @StefanKarpinski hadn't just reverted a feature addition.

This switches from using Cygwin symlinks, which only Cygwin applications can read, to NTFS junctions (as we're using in the MSYS build) for share/julia/base, share/julia/doc, and share/julia/examples, and share/julia/test. This allows the from-source Julia build in a Cygwin-to-MinGW cross-compile to reference files with the same paths relative to JULIA_HOME as a binary install.

[JeffBezanson]

Strikes me as more of a fix than a new feature.

[Keno]

Agreed. LGTM.

[tkelman]

Alright, sorry for the noise then. Once I finish adopting the p7zip Cygwin package and getting it uploaded to 64-bit Cygwin (needed for make dist), I'll add a section to README.windows.md documenting the Cygwin-to-MinGW cross-compile steps and required packages. It's been working reliably for me, and IMO it's a bit easier to set up than MSYS2.