Ubuntu xenial first dependencies

Add update and upgrade commands to enable the installation of the first dependencies on ubuntu xenial. If those are not executed some packages can not be found.

doc/build-windows.md

@@ -62,6 +62,8 @@ installing the toolchain will be different.
 
 First, install the general dependencies:
 
+    sudo apt update
+    sudo apt upgrade
     sudo apt install build-essential libtool autotools-dev automake pkg-config bsdmainutils curl git
 
 A host toolchain (`build-essential`) is necessary because some dependency