[PATCH] Put HEADLESS definition under LIBO namespace

[Riccardo Magliocchetti]

Hello,

tml reminds us that use a generic and potentially clash-prone word
like HEADLESS as preprocessor defintion is pretty lame.
He suggests to put it under the already used LIBO namespace.
To sum it up let's do a s/HEADLESS/LIBO_HEADLESS/g.

This has been done manually fixing the occurencies of HEADLESS reported 
by `git grep HEADLESS`.

Please review

-- 
Riccardo Magliocchetti