CMakeLists: use a fix for atomics on macOS PPC, not PPC in general

build/CMakeLists.txt

@@ -176,14 +176,12 @@ if(WITH_THREADSANITIZER)
   endif()
 endif()
 
-
-# Enable usage of STD's Atomic instead of Boost's on PowerPC
-# For more information refer to https://github.com/PurpleI2P/i2pd/issues/1726#issuecomment-1306335111
-if(ARCHITECTURE MATCHES "ppc")
+# Use std::atomic instead of GCC builtins on macOS PowerPC:
+# For more information refer to: https://github.com/PurpleI2P/i2pd/issues/1726#issuecomment-1306335111
+if(APPLE AND CMAKE_OSX_ARCHITECTURES MATCHES "ppc")
   add_definitions(-DBOOST_SP_USE_STD_ATOMIC)
 endif()
 
-
 # libraries
 set(THREADS_PREFER_PTHREAD_FLAG ON)
 find_package(Threads REQUIRED)