Migrate to premake5

[Itms]

We're currently using a modified version of premake4 to create our workspaces.

Premake5 is ​currently being developed and should reach a stable state soon, so we should try to start a soft migration by experimentally using their alphas along with our current version, and drop the latter when they release premake5.

Doing so will allow us:

• to generate project files for VS 2015, which we cannot use right now.
• to stop using a modified premake (see #1518), because premake5 allows us to create custom modules, which is a much more maintainable solution.

NB: They moved from BitBucket to GitHub so most of the links in the other ticket are broken, and I think one pull request vanished.

What has to be done:

• Change a bunch of things (really a no-brainer, took me 10 minutes) as explained here: ​https://github.com/premake/premake-core/wiki/Migrating-From-4.x
• Rewrite our customizations as a module (it's the big awesome change in premake5) as explained here: ​https://github.com/premake/premake-core/wiki/Extending-Premake
• If some of our fixes are not yet included and are still needed (read: if regressions appear), rewrite the fixes and make a pull request while they're in alpha and active
• If time permits: take a look at the new features and see if we can use them to improve our scripts (​https://github.com/premake/premake-core/wiki/What's-New-in-5.0); also see if our TODO/FIXME are still relevant

I tried to give it a go but I am not fluent in Lua and don't know much about all of this building stuff. The module system seems powerful and well documented, so maybe it will be really easy for people working on it before (CCing Yves and leper) to write those modules.

[Zsolt Dollenstein]

7zipped patch works on windows with vs2013 and premake5.0.0-alpha7