It's not fully implemented yet, for example `@FOO@` syntax is not handled.
This branch makes WriteFileStep use the same hashing algorithm as our cache system, reducing the divergence between zig build and the cache system.
This API converts a config.h.in file into config.h. This is useful when introducing a build.zig file to an existing C/C++ project that is configured with autotools or cmake. The cmake syntax is not implemented yet.
