Subject: bug#29814: impossible to pass spaces in GUIX_BUILD_OPTIONS

From: Martin Castillo <[email-masked]>
Date: Fri, 22 Dec 2017 18:58:06 +0100

guix/util.scm:(arguments-from-envirenment-variable) uses
char-set:graphic to split the string. this makes it impossible to pass
spaces in the arguments.

This makes it impossible to pass more than one substitute-url via the
environment.