Build should not rely on SPIRV-Tools CHANGES, but the generated file.

Shaderc build is relying on the format of the SPIRV-Tools `CHANGES` file, which is hand-made. Shaderc should rely on the generated file instead, `build-version.inc`, which has a format which is supposed to be parsed, hence stable.

Because of a change of release process, the `CHANGES` file won't reflect the exact version number for SPIRV-Tools anymore, so this might break shaderc.

(See https://github.com/KhronosGroup/SPIRV-Tools/pull/5067)