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>
Merge pull request #1106 from bendavid/py8200final_73x

update to pythia 8.200 (official release) (73x)
Merge pull request #1109 from bendavid/py8200final_53x

Backport of Pythia 8.200 final (53x)