fix: -fobjc-arc with GCC (#5632)

libtransmission/CMakeLists.txt

@@ -12,7 +12,7 @@ check_symbol_exists(SO_REUSEPORT "sys/types.h;sys/socket.h" HAVE_SO_REUSEPORT)
 
 add_compile_options(
     # equivalent of XCODE_ATTRIBUTE_CLANG_ENABLE_OBJC_ARC YES for this directory
-    $<$<AND:$<BOOL:${APPLE}>,$<COMPILE_LANGUAGE:C,CXX>>:-fobjc-arc>)
+    $<$<AND:$<BOOL:${APPLE}>,$<CXX_COMPILER_ID:AppleClang,Clang>,$<COMPILE_LANGUAGE:C,CXX>>:-fobjc-arc>)
 
 add_library(${TR_NAME} STATIC)
 
@@ -201,10 +201,10 @@ tr_allow_compile_if(
         watchdir-inotify.cc
     [=[[WITH_KQUEUE]]=]
         watchdir-kqueue.cc
-    [=[[APPLE]]=]
+    [=[[APPLE AND CMAKE_CXX_COMPILER_ID MATCHES "Clang"]]=]
         tr-assert.mm
         utils.mm
-    [=[[NOT APPLE]]=]
+    [=[[NOT (APPLE AND CMAKE_CXX_COMPILER_ID MATCHES "Clang")]]=]
         tr-assert.cc
     [=[[WIN32]]=]
         file-win32.cc

libtransmission/utils.cc

@@ -287,7 +287,7 @@ double tr_getRatio(uint64_t numerator, uint64_t denominator)
 
 // ---
 
-#ifndef __APPLE__
+#if !(defined(__APPLE__) && defined(__clang__))
 
 std::string tr_strv_convert_utf8(std::string_view sv)
 {