Emscripten target specs are conditional on the host platform

[brson]

In this PR, to make emscripten work on windows, the target specs are conditionalized to use emcc.bat as the linker, only on windows hosts.

In practice this should be perfectly fine, but I don't think it's correct for the target spec to vary like that and there should be some way to express the spec statically that works on both platforms.`

cc @schulzch

[schulzch]

Emscripten provides three files per command, e.g.:

• emcc python 2.x and 3.x wrapper which invokes emcc.py using python 2.x
• emcc.bat invokes emcc.py using python
• emcc.pycontains the actual code

I think having a host platform dependent switch is not wrong, because that is how script commands are usually wrapped on windows - the Node.js package manager npm creates wrapper *.cmd files transparently on windows.

The only other solution I can think of would be to enable shell expansion, i.e., to have windows append *.exe, *.cmd, *.bat, etc., which could imply a performance penalty (?).

[alexcrichton]

Nowadays we even have more special treatment of emcc.bat, so I think this may just be a necessary evil for now :(