Add abi_tag once it's in clang, see https://llvm.org/bugs/show_bug.cgi?id=23529 and http://reviews.llvm.org/D12834
ROOT-8186 Strange message when starting PROOF lite
ROOT-8192 ROOT on most Recent UBUNTU versions, with gcc 5.3
ROOT-8262 Segfault in FastCall in PyROOT with gcc61 with new ABI