Add shared library variant for libshaderc

[nsubtil]

Adds target shaderc_shared, which builds libshaderc as a shared library. This
is required for shipping the library on Windows, where static libraries are not
feasible.

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[antiagainst]

Hi @nsubtil, do you still intended to contribute this PR? Could you please resolve the conflicts and make it pass the tests? Thanks!

[nsubtil]

Yes, I'm still interested, but I need to do our due diligence for the CLA first. This had fallen off my radar, thanks for pinging me. I'll try to get this resolved soon.

…

On Mon, Jul 24, 2017 at 9:36 PM, Lei Zhang ***@***.***> wrote: Hi @nsubtil <https://github.com/nsubtil>, do you still intended to contribute this PR? Could you please resolve the conflicts and make it pass the tests? Thanks! — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#342 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AF66M-1kKN978j7e0iZXj_Mn1EAMuskDks5sRXDagaJpZM4NplFq> .

[antiagainst]

Cool, thanks! :)

[Kangz]

I'm also interested in this. On Windows there will be an additional SHADERC_EXPORT mechanism needed to make shaderc dlls work.

[Kangz]

@nsubtil I'm picking this up.

[nsubtil]

Thanks @Kangz. I'll keep an eye out for updates. Happy to help if I can, though I haven't really had much time for this in the past weeks.

[googlebot]

CLAs look good, thanks!

[Kangz]

@nsubtil, #366 landed so this PR could be closed.

[nsubtil]

Agreed. Thanks for the help on this!