Add SPDX license identifier to the compiler source code

[axic]

Since we advocate for this in Solidity programs, we should bite the bullet and also include it in all the compiler source code 🙊

[sacgrover]

Hey,
I would like to work on it. Do you have any more information about it?

[chriseth]

@sacgrover thanks for wanting to help out! I think what @axic meant is adding the appropriate

// SPDX-License-Identifier: GPL-3.0

to all source files of the compiler that are GPL-3-licensed.

[sacgrover]

Okay, Thanks. I will dig into it more. Please assign this to me.

[sacgrover]

One Question, SPDX-License-Identifier will always be GPL-3.0 or it can have different value?
and when we say source files. Does it mean abis json that get created?

[chriseth]

If the license snippet in that file is gpl, then it is gpl. I think we might also have some mit files.