clover: make llvm >= 3.5.0 and c++11 mandatory

clover: make llvm >= 3.5.0 and c++11 mandatory

Clover not longer compile with llvm <= 3.5.0 since e1d363b3.
e1d363b3 implies c++11 and llvm 3.5.0 CXXFLAGS provided it.
No one seems to have noticed it, it's now official.

Acked-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>