CMake: Add explicit check for atomic lib, and link it if necessary

CMakeLists.txt

@@ -125,6 +125,7 @@ configure_package_config_file("${JUCE_CMAKE_UTILS_DIR}/JUCEConfig.cmake.in"
 install(FILES "${JUCE_BINARY_DIR}/JUCEConfigVersion.cmake"
               "${JUCE_BINARY_DIR}/JUCEConfig.cmake"
               "${JUCE_CMAKE_UTILS_DIR}/JUCEHelperTargets.cmake"
+              "${JUCE_CMAKE_UTILS_DIR}/JUCECheckAtomic.cmake"
               "${JUCE_CMAKE_UTILS_DIR}/JUCEUtils.cmake"
               "${JUCE_CMAKE_UTILS_DIR}/LaunchScreen.storyboard"
               "${JUCE_CMAKE_UTILS_DIR}/PIPAudioProcessor.cpp.in"

extras/Build/CMake/JUCECheckAtomic.cmake

@@ -0,0 +1,82 @@
+# ==============================================================================
+#
+#  This file is part of the JUCE library.
+#  Copyright (c) 2020 - Raw Material Software Limited
+#
+#  JUCE is an open source library subject to commercial or open-source
+#  licensing.
+#
+#  By using JUCE, you agree to the terms of both the JUCE 6 End-User License
+#  Agreement and JUCE Privacy Policy (both effective as of the 16th June 2020).
+#
+#  End User License Agreement: www.juce.com/juce-6-licence
+#  Privacy Policy: www.juce.com/juce-privacy-policy
+#
+#  Or: You may also use this code under the terms of the GPL v3 (see
+#  www.gnu.org/licenses).
+#
+#  JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
+#  EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
+#  DISCLAIMED.
+#
+# ==============================================================================
+
+function(_juce_create_atomic_target target_name)
+    add_library("${target_name}" INTERFACE)
+    add_library("juce::${target_name}" ALIAS "${target_name}")
+
+    if(NOT (CMAKE_SYSTEM_NAME STREQUAL "Linux"))
+        return()
+    endif()
+
+    set(test_file_contents
+        [[
+            #include <atomic>
+
+            int main (int argc, char** argv)
+            {
+                std::atomic<long long> ll { static_cast<long long> (argc) };
+                ll ^= 1;
+                return static_cast<int> (ll);
+            }
+        ]])
+
+    string(RANDOM LENGTH 16 random_file_string)
+    set(test_file_name "${CMAKE_CURRENT_BINARY_DIR}/check_atomic_${random_file_string}.cpp")
+
+    string(RANDOM LENGTH 16 random_dir_string)
+    set(test_bindir "${CMAKE_CURRENT_BINARY_DIR}/check_atomic_dir_${random_dir_string}")
+
+    file(WRITE "${test_file_name}" "${test_file_contents}")
+
+    try_compile(compile_result "${test_bindir}" "${test_file_name}"
+        OUTPUT_VARIABLE test_build_output_0
+        CXX_STANDARD 11
+        CXX_STANDARD_REQUIRED TRUE
+        CXX_EXTENSIONS FALSE)
+
+    if(NOT compile_result)
+        try_compile(compile_result "${test_bindir}" "${test_file_name}"
+            OUTPUT_VARIABLE test_build_output_1
+            LINK_LIBRARIES atomic
+            CXX_STANDARD 11
+            CXX_STANDARD_REQUIRED TRUE
+            CXX_EXTENSIONS FALSE)
+
+        if (NOT compile_result)
+            message(FATAL_ERROR
+                "First build output:\n"
+                "${test_build_output_0}"
+                "\n\nSecond build output:\n"
+                "${test_build_output_1}"
+                "\n\nJUCE requires support for std::atomic, but this system cannot "
+                "successfully compile a program which uses std::atomic. "
+                "You may need to install a dedicated libatomic package using your "
+                "system's package manager.")
+        endif()
+
+        target_link_libraries("${target_name}" INTERFACE atomic)
+    endif()
+
+    file(REMOVE "${test_file_name}")
+endfunction()

extras/Build/CMake/JUCEUtils.cmake

@@ -123,6 +123,9 @@ set(JUCE_CMAKE_UTILS_DIR ${CMAKE_CURRENT_LIST_DIR}
     CACHE INTERNAL "The path to the folder holding this file and other resources")
 
 include("${JUCE_CMAKE_UTILS_DIR}/JUCEHelperTargets.cmake")
+include("${JUCE_CMAKE_UTILS_DIR}/JUCECheckAtomic.cmake")
+
+_juce_create_atomic_target(juce_atomic_wrapper)
 
 # Tries to discover the target platform architecture, which is necessary for
 # naming VST3 bundle folders correctly.
@@ -518,6 +521,8 @@ function(juce_add_module module_path)
     if(${module_name} STREQUAL "juce_core")
         _juce_add_standard_defs(${module_name})
 
+        target_link_libraries(juce_core INTERFACE juce::juce_atomic_wrapper)
+
         if(CMAKE_SYSTEM_NAME STREQUAL "Android")
             target_sources(juce_core INTERFACE "${ANDROID_NDK}/sources/android/cpufeatures/cpu-features.c")
             target_include_directories(juce_core INTERFACE "${ANDROID_NDK}/sources/android/cpufeatures")