Index: cmake/modules/CheckAtomic.cmake
===================================================================
--- /dev/null
+++ cmake/modules/CheckAtomic.cmake
@@ -0,0 +1,106 @@
+# atomic builtins are required for threading support.
+
+INCLUDE(CheckCXXSourceCompiles)
+
+# Sometimes linking against libatomic is required for atomic ops, if
+# the platform doesn't support lock-free atomics.
+
+function(check_working_cxx_atomics varname)
+  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+  set(CMAKE_REQUIRED_FLAGS "-std=c++11")
+  CHECK_CXX_SOURCE_COMPILES("
+#include <atomic>
+std::atomic<int> x;
+int main() {
+  return x;
+}
+" ${varname})
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+endfunction(check_working_cxx_atomics)
+
+function(check_working_cxx_atomics64 varname)
+  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+  set(CMAKE_REQUIRED_FLAGS "-std=c++11 ${CMAKE_REQUIRED_FLAGS}")
+  CHECK_CXX_SOURCE_COMPILES("
+#include <atomic>
+#include <cstdint>
+std::atomic<uint64_t> x (0);
+int main() {
+  uint64_t i = x.load(std::memory_order_relaxed);
+  return 0;
+}
+" ${varname})
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+endfunction(check_working_cxx_atomics64)
+
+
+# This isn't necessary on MSVC, so avoid command-line switch annoyance
+# by only running on GCC-like hosts.
+if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
+  # First check if atomics work without the library.
+  check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITHOUT_LIB)
+  # If not, check if the library exists, and atomics work with it.
+  if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
+    check_library_exists(atomic __atomic_fetch_add_4 "" HAVE_LIBATOMIC)
+    if( HAVE_LIBATOMIC )
+      list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
+      check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITH_LIB)
+      if (NOT HAVE_CXX_ATOMICS_WITH_LIB)
+	message(FATAL_ERROR "Host compiler must support std::atomic!")
+      endif()
+    else()
+      message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
+    endif()
+  endif()
+endif()
+
+# Check for 64 bit atomic operations.
+if(MSVC)
+  set(HAVE_CXX_ATOMICS64_WITHOUT_LIB True)
+else()
+  check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITHOUT_LIB)
+endif()
+
+# If not, check if the library exists, and atomics work with it.
+if(NOT HAVE_CXX_ATOMICS64_WITHOUT_LIB)
+  check_library_exists(atomic __atomic_load_8 "" HAVE_CXX_LIBATOMICS64)
+  if(HAVE_CXX_LIBATOMICS64)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
+    check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITH_LIB)
+    if (NOT HAVE_CXX_ATOMICS64_WITH_LIB)
+      message(FATAL_ERROR "Host compiler must support std::atomic!")
+    endif()
+  else()
+    message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
+  endif()
+endif()
+
+## TODO: This define is only used for the legacy atomic operations in
+## llvm's Atomic.h, which should be replaced.  Other code simply
+## assumes C++11 <atomic> works.
+CHECK_CXX_SOURCE_COMPILES("
+#ifdef _MSC_VER
+#include <Intrin.h> /* Workaround for PR19898. */
+#include <windows.h>
+#endif
+int main() {
+#ifdef _MSC_VER
+        volatile LONG val = 1;
+        MemoryBarrier();
+        InterlockedCompareExchange(&val, 0, 1);
+        InterlockedIncrement(&val);
+        InterlockedDecrement(&val);
+#else
+        volatile unsigned long val = 1;
+        __sync_synchronize();
+        __sync_val_compare_and_swap(&val, 1, 0);
+        __sync_add_and_fetch(&val, 1);
+        __sync_sub_and_fetch(&val, 1);
+#endif
+        return 0;
+      }
+" LLVM_HAS_ATOMICS)
+
+if( NOT LLVM_HAS_ATOMICS )
+  message(STATUS "Warning: LLVM will be built thread-unsafe because atomic builtins are missing")
+endif()
Index: cmake/modules/LLDBStandalone.cmake
===================================================================
--- cmake/modules/LLDBStandalone.cmake
+++ cmake/modules/LLDBStandalone.cmake
@@ -2,7 +2,7 @@
 # standalone project, using LLVM as an external library:
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   project(lldb)
-  cmake_minimum_required(VERSION 2.8.12.2)
+  cmake_minimum_required(VERSION 3.4.3)
 
   if (POLICY CMP0022)
     cmake_policy(SET CMP0022 NEW) # automatic when 2.8.12 is required
@@ -86,7 +86,7 @@
 
   include(AddLLVM)
   include(HandleLLVMOptions)
-  include(CheckAtomic)
+  include(cmake/modules/CheckAtomic.cmake)
 
   if (PYTHON_EXECUTABLE STREQUAL "")
     set(Python_ADDITIONAL_VERSIONS 3.5 3.4 3.3 3.2 3.1 3.0 2.7 2.6 2.5)