WIP: Add a manifest file for the sake of dlopening Python extensions on Windows

[tkelman]

fixes JuliaPy/PyCall.jl#87, cc @stevengj @AndyGreenwell @ihnorton

This needs some testing. I may need to use a different manifest on 32 bit, and I should probably also test against Python 3.x to see what happens there.

[ihnorton]

Nice work. Are we actually missing the manifest, or is this a work-around for Python not doing the right thing?

[stevengj]

@ihnorton, both. It seems like the manifest is something we should generically be including, although some of the Python modules could also potentially load libraries in a more robust way.

[tkelman]

We don't need the manifest because we don't use the versioned msvcrt or side by side assemblies. Python does some activation context stuff to deal with this, but they incorrectly leave the manifest out when they build .pyd files and load them with ctypes. So programs built against a different c runtime than python need to jump through otherwise unnecessary hoops.

[tkelman]

It looks like this also fixes things on 32 bit Python 2.7, and this issue isn't present for Python 3(.5?).

[tkelman]

Apparently this has been causing problems for people on 0.4.6 who don't have the msvc runtimes already installed. I think the thing to do is build a second julia-manifest.exe (open to better names) that has this included for PyCall users to run, but not include it in the default julia.exe.

[stevengj]

@tkelman, could you clarify the problems on 0.4.6? How could the necessary runtime not be installed? Don't we need an crt library to run julia?

[tkelman]

We need the unversioned msvcrt.dll which comes pre-installed on Windows. msvcr90.dll, which Python 2.7 needs, does not. Many people will already have a copy somewhere from some other program (especially if you've installed a copy of python), but not everyone.

[stevengj]

But doesn't the manifest specify the unversioned one? Or if we want to specify the versioned one, shouldn't we ship it with Julia? I'm still confused about what exactly the manifest does.

[tkelman]

The manifest specifies the versioned one. ctypes in Python doesn't properly set up the "activation context" when loading pyd files for extension modules, it assumes the versioned msvc runtime is already loaded in the process (usually by Python itself). Julia doesn't set up the activation context for the versioned runtime, unless we add this manifest - but apparently the manifest also adds a dependency on the versioned runtime. We can't ship the versioned MSVC runtime because it isn't GPL compatible.