Index: cmake/Modules/CheckLibcxxAtomic.cmake
===================================================================
--- cmake/Modules/CheckLibcxxAtomic.cmake
+++ cmake/Modules/CheckLibcxxAtomic.cmake
@@ -1,4 +1,4 @@
-INCLUDE(CheckCXXSourceCompiles)
+INCLUDE(CheckCSourceCompiles)
 
 # Sometimes linking against libatomic is required for atomic ops, if
 # the platform doesn't support lock-free atomics.
@@ -9,17 +9,16 @@
 
 function(check_cxx_atomics varname)
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "-std=c++11 -nostdinc++ -isystem ${LIBCXX_SOURCE_DIR}/include")
+  set(CMAKE_REQUIRED_FLAGS "-std=c11")
   if (${LIBCXX_GCC_TOOLCHAIN})
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
   endif()
-  check_cxx_source_compiles("
-#include <cstdint>
-#include <atomic>
-std::atomic<uintptr_t> x;
-std::atomic<uintmax_t> y;
+  check_c_source_compiles("
+#include <stdatomic.h>
+atomic_uintptr_t x;
+atomic_uintmax_t y;
 int main() {
-  return x + y;
+  return atomic_load(&x) + atomic_load(&y);
 }
 " ${varname})
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})