gui/build.sh: properly respect user variables and also upstream some other Gentoo patches

This ensures proper building when cross-compiling and allows users to use different toolchain tools if desired

On Gentoo the ip command is in /bin/ and not in /sbin/

The conf.py needs sphinx.ext.autodoc because it is using external plugins (alabaster)

And finally I added a note to the README about the ebuild in Gentoo.

Bug: https://bugs.gentoo.org/814017 Closes: #550 (closed) Signed-off-by: Andrew Ammerlaan andrewammerlaan@gentoo.org

docs/conf.py

@@ -39,6 +39,7 @@ release = '0.19.7'
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
 extensions = [
+    'sphinx.ext.autodoc',
 ]
 
 # Add any paths that contain templates here, relative to this directory.