enable boostrapping with Lua5.2

Hi,

Here is a first step to make LuaDist running with Lua5.2.

This commit enables to boostrap LuaDist with Lua5.2.
For now it works if you checkout the "master" branch for each boostrap.git submodules, except for luasocket where you need to checkout the "unstable" branch.

It is worth noting the second boostrap phase (i.e. LuaDist building with itself) actually builds LuaDist with dependency versions than may differ from the dependency versions used to fill the _boostrap directory (due to the install getting dependency sources by itself).
Consequently _install binaries files versions may differ from _boostrap binaries files versions.

Do you plan to upgrade to 5.2 into master or to create a separated Lua5.2 branch ?

Thanks in advance for your answer,

  Cyril Romain