[bug#35936] [PATCH] README: add Guix System configure hints

[Robert Vollmert]

> On 28. May 2019, at 12:34, Ricardo Wurmus <address@hidden> wrote:
>> 
>> +    On Guix System, this is '/var' by default. Note that you will need to
>> +    re-run the 'configure' script after updating Guix System.
>> +
> 
> Why would re-running the configure script be required after updating?
> 
> This section is about building Guix on a system that already has some
> version of Guix to reduce the complexity of establishing a suitable
> build environment.  I don’t think reconfiguring Guix System is relevant
> here.

It’s my attempt to help people not be as confused as I was. Maybe there
are better ways to achieve this? The two situations:

1. I followed the README instructions to build guix, and was quite confused
by the `localstatedir` business. I seem to be doing the most straightforward
thing, but have to dig through documentation/internet to continue since I
have no idea what/where localstatedir is. (I still don’t, and have been
doing quite fine without the knowledge.)

2. I ran guix pull && guix package -u. And afterwards, the guix development
environment was broken in an obscure way. Re-running guix environment guix
didn’t help. It turns out configure stores some full paths to core utilities,
such as

$ grep ^MKDIR_P config.status 
MKDIR_P='/gnu/store/lkrj6zifhk5dk888713dqyzfwk5bich0-profile/bin/mkdir -p’

To me it’s entirely unexpected that after a small OS update my dev environment
should break in this way.