Update build instructions for Ubuntu trusty

[Andy Alt]

While the required dependency versions are listed in the ​Linux section, the build instructions for ​trusty show

sudo apt-get install build-essential libboost-dev libboost-filesystem-dev   \
    libcurl4-gnutls-dev libenet-dev libgloox-dev libicu-dev    \
    libminiupnpc-dev libnspr4-dev libnvtt-dev libogg-dev libopenal-dev   \
    libpng-dev libsdl2-dev libvorbis-dev libwxgtk3.0-dev libxcursor-dev  \
    libxml2-dev subversion zlib1g-dev

which would install a few outdated dependencies from the trusty repo, such as libgloox-dev and libboost-dev, and would result in an incomplete build.

On trusty, prebuilt packages can be installed by using the 0ad PPA, like this:

sudo add-apt-repository -y ppa:wfg/0ad
sudo apt-get update

sudo apt-get install -y \
    libsodium-dev libgloox-dev \
libenet-dev libboost1.58-dev libboost-filesystem1.58-dev

But how to separate the trusty section on the build instructions with other versions of ubuntu so things don't get cluttered? And for how long does 0ad need to support trusty?

[historic_bruno]

Note: Trusty reached EOL 2019-04-17.

[Stan]

Trusty is now too old, and probably doesn't even have a good gcc version.