Xfitter 42

I have added the suggestions from Sasha, on my setup they work, though I can't test the c+11 option in configure.ac because that flag is alrady getting added from the -config commands from some of my other installed packages.

added 252 commits

• 116bc94e...9da64f76 - 251 commits from branch master
• c069b0ec - Merge branch 'master' into 'XFITTER-42'

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mentioned in commit 6ccd70c7