Motivation for this change

I don't know if there's much interest in this. I suppose it will make adding a eventual gcc7 easier.

It's almost a copypaste of gcc6, except for:

38a39
> , flex ? null
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< let version = "6.2.0";
---
> let version = "7-20161211";
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<       [ ../use-source-date-epoch.patch ]
---
>       [ ]
218,219c219,220
<     url = "mirror://gnu/gcc/gcc-${version}/gcc-${version}.tar.bz2";
<     sha256 = "1idpf43988v1a6i8lw9ak1r7igcfg1bm5kn011iydlr2qygmhi4r";
---
>     url = "mirror://gcc/snapshots/${version}/gcc-${version}.tar.bz2";
>     sha256 = "114rrrm5d5cawmr3161d2wgjlzkb9l1imj1p7fnypwz7y85l1661";
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<   buildInputs = [ gmp mpfr libmpc libelf ]
---
>   buildInputs = [ gmp mpfr libmpc libelf flex ]

https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=e3e8c48c4a494d9da741c1c8ea6c4c0b7c4ff934 seems that the SOURCE_DATE_EPOCH patch was included. Also there's a file gengtype-lex.l that requires flex.

Tried some c++17 things and it worked.

I suspect there's some build problem, maybe parallel related, because I tried compiling once and it failed, tried again and it worked.

edit: just remembered not to add maintainers.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.