pythia8: do not use -Qunused-arguments on GCC

By default Pythia8 assumes that on Darwin LLVM/Clang will be used and
-Qunused-arguments option is passed. Such option is not supported
by GCC. Modify configure script to check for Clang.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
update to pythia 8.200 (official release)
Merge pull request #1118 from bendavid/py8200final_71x

Pythia 8.200 final (71x)
Merge remote branch 'origin/IB/CMSSW_7_1_X/stable' into IB/CMSSW_7_0_X/stable