Use -latomic when using clang (server platform)

version of c9b3a00a63fafa546bf402e32fd21ebe7f274b43 for the server
platform

platform/server/detect.py

@@ -93,6 +93,7 @@ def configure(env):
             env["CC"] = "clang"
             env["CXX"] = "clang++"
         env.extra_suffix = ".llvm" + env.extra_suffix
+        env.Append(LIBS=["atomic"])
 
     if env["use_coverage"]:
         env.Append(CCFLAGS=["-ftest-coverage", "-fprofile-arcs"])