Fixed GCC mem-access warnings (#2565)

+ using "if defined" more consistently for Clang.

imgui_internal.h

@@ -36,15 +36,17 @@ Index of this file:
 #include <math.h>       // sqrtf, fabsf, fmodf, powf, floorf, ceilf, cosf, sinf
 #include <limits.h>     // INT_MIN, INT_MAX
 
+// Visual Studio warnings
 #ifdef _MSC_VER
 #pragma warning (push)
 #pragma warning (disable: 4251) // class 'xxx' needs to have dll-interface to be used by clients of struct 'xxx' // when IMGUI_API is set to__declspec(dllexport)
 #endif
 
-#ifdef __clang__
+// Clang/GCC warnings with -Weverything
+#if defined(__clang__)
 #pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wunused-function"                // for stb_textedit.h
-#pragma clang diagnostic ignored "-Wmissing-prototypes"             // for stb_textedit.h
+#pragma clang diagnostic ignored "-Wunused-function"        // for stb_textedit.h
+#pragma clang diagnostic ignored "-Wmissing-prototypes"     // for stb_textedit.h
 #pragma clang diagnostic ignored "-Wold-style-cast"
 #if __has_warning("-Wzero-as-null-pointer-constant")
 #pragma clang diagnostic ignored "-Wzero-as-null-pointer-constant"
@@ -52,6 +54,11 @@ Index of this file:
 #if __has_warning("-Wdouble-promotion")
 #pragma clang diagnostic ignored "-Wdouble-promotion"
 #endif
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#if __GNUC__ >= 8
+#pragma GCC diagnostic ignored "-Wclass-memaccess"          // warning: 'memset/memcpy' clearing/writing an object of type 'xxxx' with no trivial copy-assignment; use assignment or value-initialization instead
+#endif
 #endif
 
 //-----------------------------------------------------------------------------
@@ -1645,8 +1652,10 @@ extern void                 ImGuiTestEngineHook_ItemInfo(ImGuiContext* ctx, ImGu
 #define IMGUI_TEST_ENGINE_ITEM_INFO(_ID, _LABEL, _FLAGS)  do { } while (0)
 #endif
 
-#ifdef __clang__
+#if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
 
 #ifdef _MSC_VER