[doc] move all legacy host information into a dedicated section

It's just distracting to have legacy host application information
interleaved with the other stuff when it's deprecated.

doc/build_host.rst

@@ -0,0 +1,130 @@
+.. _host_building:
+
+Legacy Host Application build
+#############################
+
+.. warning::
+
+   The Host Application is a legacy server application. New installations should
+   use the prebuilt :doc:`Looking Glass IDD <install_idd>`. These build
+   instructions are retained for the complete matching B7 stack described in
+   :ref:`legacy_host_policy`.
+
+.. note::
+
+   `Host` in this context refers to the `Looking Glass (LG) host server application`,
+   to which the LG client connects to, it thus does not refer to the `host OS`!
+
+   A common setup is to have a `host OS` running a `guest VM`. In such a setup
+   the `client application` runs on the `host OS`; and the `host application`
+   on the `guest VM`.
+
+   The term `Host application` was chosen over `Guest application`, because LG
+   can be run in a VM to VM configuration, in which case both Host Application and
+   Client Application are run inside VM Guests.
+
+These instructions help you build the host yourself from the
+:ref:`downloaded source code <download_source>`.
+
+.. warning::
+   :name: dont-build-the-host
+
+   Building the host from source code is not recommended for most purposes,
+   and should only be attempted by users who are prepared to handle issues
+   on their own. Please download the pre-built binary installers from
+   https://looking-glass.io/downloads for stability, and increased support.
+
+   .. note::
+
+      The pre-built binaries also include NvFBC support built in, which is
+      only available to current Nvidia SDK license holders, and cannot
+      be enabled when building the host without also having a license.
+
+   (`link <#dont-build-the-host>`_)
+
+.. _host_win_on_win:
+
+For Windows on Windows
+~~~~~~~~~~~~~~~~~~~~~~
+
+1. Download and install msys2 x86_64 from
+   `http://www.msys2.org/ <http://www.msys2.org/>`__ following the setup
+   instructions provided
+
+2. Run the MSYS2 shell
+
+3. Download build dependencies with pacman
+
+.. code:: bash
+
+   pacman -Fy
+   pacman -Sy git make mingw-w64-x86_64-gcc mingw-w64-x86_64-cmake
+
+4. Change directory to the source tree with ``cd``
+
+5. Configure the project and build it
+
+.. code:: bash
+
+   mkdir host/build
+   cd host/build
+   cmake -G "MSYS Makefiles" ..
+   make
+
+.. _host_linux_on_linux:
+
+For Linux on Linux
+~~~~~~~~~~~~~~~~~~
+
+Make a ``host/build`` directory, then run ``cmake``
+
+.. code:: bash
+
+   mkdir host/build
+   cd host/build
+   cmake ..
+   make
+
+.. _host_win_cross_on_linux:
+
+For Windows cross compiling on Linux
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Like :ref:`host_linux_on_linux`, but using the mingw64 toolchain to
+cross-compile a Windows ``.exe`` file.
+
+.. code:: bash
+
+   mkdir host/build
+   cd host/build
+   cmake -DCMAKE_TOOLCHAIN_FILE=../toolchain-mingw64.cmake ..
+   make
+
+.. _host_build_installer:
+
+Building the Windows installer
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+1. :ref:`Build <host_win_cross_on_linux>` the host on Linux.
+
+2. Install ``nsis``
+
+.. code:: bash
+
+   apt-get install nsis
+
+3. Use ``makensis`` to build the installer.
+
+.. code:: bash
+
+   cd host/build/platform/Windows
+   makensis installer.nsi
+
+.. _host_questions:
+
+This will build ``looking-glass-host-setup.exe`` under
+``host/build/platform/Windows/looking-glass-host-setup.exe``
+
+.. seealso::
+
+   :ref:`Installing the Host <host_install>`