Disable systemd when building on macOS using CMake (#953)

systemd is not available on macOS - should be disabled in CMake 

Test plan:
- Run cmake in a clean folder
- Run make
- Observe that compilation error about missing include `<systemd/sd-daemon.h>` is gone

src/apps/relay/CMakeLists.txt

@@ -75,8 +75,12 @@ else()
     list(APPEND turnserver_DEFINED TURN_NO_HIREDIS)
 endif()
 
-find_package(libsystemd)
-if(libsystemd_FOUND)
+if(NOT APPLE)
+    find_package(libsystemd)
+    if(libsystemd_FOUND)
+    else()
+        list(APPEND turnserver_DEFINED TURN_NO_SYSTEMD)
+    endif()
 else()
     list(APPEND turnserver_DEFINED TURN_NO_SYSTEMD)
 endif()