[SYCL] Append extra environment variables (#531)

When extra environment variables are set in LIT configuration append
them instead of overwriting system ones.

Author: vladimirlaz

SYCL/lit.cfg.py

@@ -85,7 +85,7 @@
     for env_pair in config.extra_environment.split(','):
         [var,val]=env_pair.split("=")
         if val:
-           llvm_config.with_environment(var,val)
+           llvm_config.with_environment(var,val,append_path=True)
            lit_config.note("\t"+var+"="+val)
         else:
            lit_config.note("\tUnset "+var)