doc: revise installation instructions

* add subsections in the list of prerequisites

* mention a C++11 compiler first

* a system installation of Boost C++ won't do it

* builds without GPU support are possible

* mention LuaJIT as an alternative to Lua

* require Sphinx ≥ 1.2

* drop section on AIX
  (We're not going to run HAL's MD package on AIX.)
packages.mk: update HALMD to 1.0-alpha2

Download the tarball from halmd.org and verify its signature.