Check if we need to link against libatomic

On some 32-bit platforms, explicit linking against libatomic is
required for atomics to be utilized.  Test that at the CMake level,
adding an import target as we did for `std::filesystem` that can
be used.

Based on the suggestion from:
#81

Author: bbockelm

CMakeLists.txt

@@ -41,6 +41,8 @@ endif()
 # Our custom Filesystem module creates the std::filesystem library target for
 # any special dependencies needed for using std::filesystem.
 find_package( Filesystem REQUIRED )
+# Similar setup for libatomic; required only on 32-bit systems
+find_package( Atomic REQUIRED )
 
 if( NOT XROOTD_EXTERNAL_TINYXML2 )
   include(FetchContent)
@@ -77,7 +79,7 @@ add_definitions( -D_FILE_OFFSET_BITS=64 )
 add_library(XrdS3Obj OBJECT src/CurlUtil.cc src/S3File.cc src/S3Directory.cc src/S3AccessInfo.cc src/S3FileSystem.cc src/AWSv4-impl.cc src/S3Commands.cc src/HTTPCommands.cc src/TokenFile.cc src/stl_string_utils.cc src/shortfile.cc src/logging.cc)
 set_target_properties(XrdS3Obj PROPERTIES POSITION_INDEPENDENT_CODE ON)
 target_include_directories(XrdS3Obj PRIVATE ${XRootD_INCLUDE_DIRS})
-target_link_libraries(XrdS3Obj ${XRootD_UTILS_LIBRARIES} ${XRootD_SERVER_LIBRARIES} CURL::libcurl OpenSSL::Crypto tinyxml2::tinyxml2 Threads::Threads std::filesystem)
+target_link_libraries( XrdS3Obj ${XRootD_UTILS_LIBRARIES} ${XRootD_SERVER_LIBRARIES} CURL::libcurl OpenSSL::Crypto tinyxml2::tinyxml2 Threads::Threads std::filesystem std::atomic )
 
 add_library(XrdS3 MODULE "$<TARGET_OBJECTS:XrdS3Obj>")
 target_link_libraries(XrdS3 XrdS3Obj)

cmake/FindAtomic.cmake

@@ -0,0 +1,57 @@
+
+# Ideas come from
+#
+#  https://gitlab.kitware.com/cmake/cmake/-/issues/17834
+#
+# But applied to the use of libatomic instead of libstdc++fs.
+# The need to do this was highlighted as part of:
+#   https://github.com/PelicanPlatform/xrootd-s3-http/pull/81
+# The original driving use case was 32-bit builds; if we
+# decide to drop those, we can rip out the target
+
+include(CheckSourceCompiles)
+
+function( check_working_cxx_atomics varname )
+  CHECK_SOURCE_COMPILES( CXX "
+    #include <cstdlib>
+    #include <atomic>
+    #include <cstdint>
+
+    int main() {
+      std::atomic<uint8_t> a1;
+      std::atomic<uint16_t> a2;
+      std::atomic<uint32_t> a3;
+      std::atomic<uint64_t> a4;
+      return a1++ + a2++ + a3++ + a4++;
+    }" ${varname}
+  )
+endfunction( check_working_cxx_atomics varname )
+
+
+check_working_cxx_atomics( CXX_ATOMIC_NO_LINK_NEEDED )
+
+set( _found FALSE )
+if( CXX_ATOMIC_NO_LINK_NEEDED )
+  set( _found TRUE )
+else()
+  check_library_exists(atomic __atomic_fetch_add_4 "" HAVE_LIBATOMIC)
+  if (HAVE_LIBATOMIC)
+    set( OLD_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} )
+    list( APPEND CMAKE_REQUIRED_LIBRARIES "atomic" )
+    check_working_cxx_atomics( HAVE_CXX_ATOMICS_WITH_LIB )
+    set( CMAKE_REQUIRED_LIBRARIES ${OLD_CMAKE_REQUIRED_LIBRARIES} )
+    set( HAVE_CXX_ATOMICS_WITH_LIB TRUE )
+    set( _found TRUE )
+  endif()
+endif()
+
+add_library( std::atomic INTERFACE IMPORTED )
+
+if( HAVE_CXX_ATOMICS_WITH_LIB )
+  set_property( TARGET std::atomic APPEND PROPERTY INTERFACE_LINK_LIBRARIES atomic )
+endif()
+
+set( Atomic_FOUND ${_found} CACHE BOOL "TRUE if we can run a program using std::atomic" FORCE )
+if( Atomic_FIND_REQUIRED AND NOT Atomic_FOUND )
+    message( FATAL_ERROR "Cannot run simple program using std::atomic" )
+endif()