app_lua: LUAJIT variable to build agains LuaJIT compiler

[vhernando]

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue #XXXX (replace XXXX with an open issue number)

Description

Added LUAJIT variable to build app_lua module against LuaJIT compiler (version 5.1). If this variable is not set users still build app_lua module against standard Lua.

E.g: $ LUAJIT="yes" make modules modules=modules/app_lua # Build against LuaJIT
$ make modules modules=modules/app_lua # Build against standard Lua.

(Warning: LuaJIT version is 5.1 so scripts prepared for higher Lua versions may not work with LuaJIT)

[miconda]

This can be merged, but a note about this compile option and the constraint with luajit version has to be added to the docs of the module.

[miconda]

For future reference, you do not have to generate the README anymore, it is generated automatically by the server couple of minutes after pushing the commit to xml file.