add -lpthread to guetzli.make

[shrx]

Fixes #14

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[robryk]

This file is generated by premake. Would you mind adding the change to premake5.lua in the list where -lz is present and regenerating this file using premake?

[shrx]

I updated the file premake5.lua, however I am not familiar with premake so I don't know the procedure to regenerate using the modification.

[robryk]

I am terribly sorry, but signing a CLA is a hard requirement for legal reasons. Likewise the requirement that you provide an address and a contact phone when doing so.

I shan't merge a patch for which that is not satisfied. Sorry again.