Use -latomic when linking whe using clang on Linux

According to the LLVM documentation when using GNU's libstdc++ clang
will not automatically link with -latomic. This is necessary since we
merged c++11 atomics support.

This fixes linking using Clang on Linux

platform/x11/detect.py

@@ -129,6 +129,7 @@ def configure(env):
             env["CC"] = "clang"
             env["CXX"] = "clang++"
         env.extra_suffix = ".llvm" + env.extra_suffix
+        env.Append(LIBS=["atomic"])
 
     if env["use_lld"]:
         if env["use_llvm"]: