When locally building the doc (aka make), CURRENT_LANG is not defined,
which results in HTML_BUILD_DIR = _build/html//, resulting in commands
& logs like the following

mkdir -p _build/html//_static
pysassc extensions/odoo_theme/static/style.scss _build/html//_static/style.css

This has no impact on linux builds, but could be unclear/confusing for some non tech
users (can it have any impact on other OS's/distros ?)

closes odoo/documentation#1095

X-original-commit: d8684e89


Signed-off-by: Antoine Vandevenne (anv) <AntoineVDV@users.noreply.github.com>
Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>