Reworked LLVM installation to provide two more options: 'packaged' and '' (the 
latter means just use whatever version is installed if possible)

Improved existing clang package support, now more resilient to border cases and 
formalized as an LLVM_VERSION variant.

Switched LLVM_VERSION to 'packaged' in the default build profile.


git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/BuildScripts@10487 7b3f36aa-e6db-49c1-8b4c-66eac1997e64