cmake: properly link against libatomic when needed

Similar to CMAKE_REQUIRED_FLAGS, CMAKE_CXX_LINK_FLAGS is not an
appropriate variable for linking against libraries.

client/CMakeLists.txt

@@ -7,7 +7,7 @@ set(CLIENT_SOURCES
     decoder/pcm_decoder.cpp
     player/player.cpp)
 
-set(CLIENT_LIBRARIES ${CMAKE_THREAD_LIBS_INIT} common)
+set(CLIENT_LIBRARIES ${CMAKE_THREAD_LIBS_INIT} ${ATOMIC_LIBRARY} common)
 
 set(CLIENT_INCLUDE
     ${Boost_INCLUDE_DIR}

cmake/CheckAtomic.cmake

@@ -108,6 +108,8 @@ endfunction(check_working_cxx_atomics64_2args)
 # First check if atomics work without the library.
 check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITHOUT_LIB)
 
+set(ATOMIC_LIBRARY "")
+
 # If not, check if the library exists, and atomics work with it.
 if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
   check_library_exists(atomic __atomic_fetch_add_4 "" HAVE_LIBATOMIC)
@@ -116,7 +118,7 @@ if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
   endif()
   if( HAVE_LIBATOMIC OR HAVE_LIBATOMIC_2ARGS )
     list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
-    set(CMAKE_CXX_LINK_FLAGS "${CMAKE_CXX_LINK_FLAGS} -latomic")
+    set(ATOMIC_LIBRARY "atomic")
     check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITH_LIB)
     if (NOT HAVE_CXX_ATOMICS_WITH_LIB)
       message(FATAL_ERROR "Host compiler must support std::atomic!")
@@ -137,7 +139,7 @@ if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
       endif()
       if( HAVE_CXX_LIBATOMICS64 OR HAVE_CXX_LIBATOMICS64_2ARGS )
         list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
-        set(CMAKE_CXX_LINK_FLAGS "${CMAKE_CXX_LINK_FLAGS} -latomic")
+        set(ATOMIC_LIBRARY "atomic")
         check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITH_LIB)
         if (NOT HAVE_CXX_ATOMICS64_WITH_LIB)
           message(FATAL_ERROR "Host compiler must support std::atomic!")

server/CMakeLists.txt

@@ -23,6 +23,7 @@ set(SERVER_SOURCES
 
 set(SERVER_LIBRARIES
     ${CMAKE_THREAD_LIBS_INIT}
+    ${ATOMIC_LIBRARY}
     common)
 
 set(SERVER_INCLUDE