build with -latomic on platforms that need it

cmake/CheckAtomic.cmake

@@ -0,0 +1,41 @@
+# - Try to find if 64-bits atomics need -latomic linking
+# Once done this will define
+#  HAVE_CXX_ATOMICS_WITHOUT_LIB - Whether atomic types work without -latomic
+
+INCLUDE(CheckCXXSourceCompiles)
+INCLUDE(CheckLibraryExists)
+
+# Sometimes linking against libatomic is required for atomic ops, if
+# the platform doesn't support lock-free atomics.
+
+function(check_working_cxx_atomics varname)
+CHECK_CXX_SOURCE_COMPILES("
+#include <atomic>
+int main() {
+	std::atomic<long long> x;
+	return std::atomic_is_lock_free(&x);
+}
+" ${varname})
+endfunction(check_working_cxx_atomics)
+
+# Check for atomic operations.
+if(MSVC)
+  # This isn't necessary on MSVC.
+  set(HAVE_CXX_ATOMICS_WITHOUT_LIB True)
+else()
+  # First check if atomics work without the library.
+  check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITHOUT_LIB)
+endif()
+
+# If not, check if the library exists, and atomics work with it.
+if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
+  check_library_exists(atomic __atomic_load_8 "" HAVE_LIBATOMIC)
+  if(NOT HAVE_LIBATOMIC)
+    message(STATUS "Host compiler appears to require libatomic, but cannot locate it.")
+  endif()
+  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()
+endif()

libtransmission/CMakeLists.txt

@@ -1,5 +1,6 @@
 include(CheckLibraryExists)
 include(CheckSymbolExists)
+include(CheckAtomic)
 
 if(ENABLE_NLS)
     check_library_exists(intl libintl_gettext "" HAVE_LIBINTL)
@@ -305,6 +306,10 @@ target_link_libraries(${TR_NAME}
         small::small
         libevent::event)
 
+if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
+    target_link_libraries(${TR_NAME} PRIVATE atomic)
+endif()
+
 if(INSTALL_LIB)
     install(
         TARGETS ${TR_NAME}