Find libatomic fix cmake magics

cmake/unix.cmake

@@ -2,6 +2,9 @@ if(NOT UNIX)
   return()
 endif()
 
+include(CheckCXXSourceCompiles)
+include(CheckLibraryExists)
+
 add_definitions(-DUNIX)
 add_definitions(-DPOSIX)
 
@@ -21,9 +24,46 @@ endif()
 
 include_directories(${LIBUV_INCLUDE_DIRS})
 
+function(check_working_cxx_atomics64 varname)
+  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;
+}
+" ${varname})
+endfunction()
+
+function(link_libatomic)
+  check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITHOUT_LIB)
+
+  if(HAVE_CXX_ATOMICS64_WITHOUT_LIB)
+    message(STATUS "Have working 64bit atomics")
+    return()
+  endif()
+
+  check_library_exists(atomic __atomic_load_8 "" HAVE_CXX_LIBATOMICS64)
+  if (HAVE_CXX_LIBATOMICS64)
+    message(STATUS "Have 64bit atomics via library")
+    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
+    check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITH_LIB)
+    if (HAVE_CXX_ATOMICS64_WITH_LIB)
+      message(STATUS "Can link with libatomic")
+      link_libraries(-latomic)
+      return()
+    endif()
+  endif()
+
+  message(FATAL_ERROR "Host compiler must support 64-bit std::atomic!")
+endfunction()
+
 if(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
   set(FS_LIB stdc++fs)
   get_filename_component(LIBTUNTAP_IMPL ${TT_ROOT}/tuntap-unix-linux.c ABSOLUTE)
+
+  link_libatomic()
 elseif(${CMAKE_SYSTEM_NAME} MATCHES "Android")
   find_library(FS_LIB NAMES c++fs c++experimental stdc++fs)
   if(FS_LIB STREQUAL FS_LIB-NOTFOUND)