cmake: sync gamemode conditionals with code after 5eec980a2d

FAILED: bin/yuzu
ld: error: unable to find library -lgamemode

FAILED: bin/yuzu-cmd
ld: error: undefined symbol: Common::Linux::StartGamemode()
>>> referenced by yuzu.cpp
>>>               src/yuzu_cmd/CMakeFiles/yuzu-cmd.dir/yuzu.cpp.o:(main)

ld: error: undefined symbol: Common::Linux::StopGamemode()
>>> referenced by yuzu.cpp
>>>               src/yuzu_cmd/CMakeFiles/yuzu-cmd.dir/yuzu.cpp.o:(main)

externals/CMakeLists.txt

@@ -189,7 +189,7 @@ if (ANDROID)
    endif()
 endif()
 
-if (CMAKE_SYSTEM_NAME STREQUAL "Linux")
+if (UNIX AND NOT APPLE)
     add_subdirectory(gamemode)
 endif()
 

src/common/CMakeLists.txt

@@ -174,7 +174,7 @@ if(ANDROID)
     )
 endif()
 
-if (CMAKE_SYSTEM_NAME STREQUAL "Linux")
+if (UNIX AND NOT APPLE)
   target_sources(common PRIVATE
     linux/gamemode.cpp
     linux/gamemode.h