Added a comment hinting at how to set IMGUI_API for shared librairies on e.g. Linux, macOS (#7806)

imconfig.h

@@ -21,10 +21,11 @@
 
 //---- Define attributes of all API symbols declarations, e.g. for DLL under Windows
 // Using Dear ImGui via a shared library is not recommended, because of function call overhead and because we don't guarantee backward nor forward ABI compatibility.
-// DLL users: heaps and globals are not shared across DLL boundaries! You will need to call SetCurrentContext() + SetAllocatorFunctions()
-// for each static/DLL boundary you are calling from. Read "Context and Memory Allocators" section of imgui.cpp for more details.
-//#define IMGUI_API __declspec( dllexport )
-//#define IMGUI_API __declspec( dllimport )
+// - Windows DLL users: heaps and globals are not shared across DLL boundaries! You will need to call SetCurrentContext() + SetAllocatorFunctions()
+//   for each static/DLL boundary you are calling from. Read "Context and Memory Allocators" section of imgui.cpp for more details.
+//#define IMGUI_API __declspec(dllexport)                   // MSVC Windows: DLL export
+//#define IMGUI_API __declspec(dllimport)                   // MSVC Windows: DLL import
+//#define IMGUI_API __attribute__((visibility("default")))  // GCC/Clang: override visibility when set is hidden
 
 //---- Don't define obsolete functions/enums/behaviors. Consider enabling from time to time after updating to clean your code of obsolete function/names.
 //#define IMGUI_DISABLE_OBSOLETE_FUNCTIONS