Fix clang 3.5 compilation

- detect c++11 compatibility at configure time
- only add Warnings if they are supported by the compiler
- cleanup messy Makefile.am
Do not use unix as a variable name

unix is used as a macro name in some headers, avoid it.
Do not redefine the default argument
Comment out functions flagged as unused by clang
Merge pull request #7 from serge-sans-paille/bugfixes/clang-build

Bugfixes/clang build