For future reference, this is what you need to do in order to install Mono and XSP on Ubuntu Server 12.04.3 LTS x64. This is about a completely blank installation, where only the SSH server role was selected during the installation of the OS.
You'll need the following commands to set up mono in /usr/local. Keep in mind that
apt-get install and
make install are going to need root privileges:
apt-get install git apt-get install build-essential pkg-config apt-get install autoconf automake libtool gettext git clone git://github.com/mono/mono.git cd mono ./autogen.sh --prefix=/usr/local make get-monolite-latest make make install
And these for XSP:
git clone git://github.com/mono/xsp.git cd xsp ./autogen.sh ./configure --prefix=/usr/local make make install
And that's it!
Update, 2013-10-27: Here's a related excellent article, which covers additionally the installation of libgdiplus.