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

mentioned in issue #550 (closed)

assigned to @kali

merged