Fixed GCC mem-access warnings (#2565)

+ using "if defined" more consistently for Clang.

imgui_draw.cpp

@@ -52,7 +52,7 @@ Index of this file:
 #endif
 
 // Clang/GCC warnings with -Weverything
-#ifdef __clang__
+#if defined(__clang__)
 #pragma clang diagnostic ignored "-Wold-style-cast"         // warning : use of old-style cast                              // yes, they are more terse.
 #pragma clang diagnostic ignored "-Wfloat-equal"            // warning : comparing floating point with == or != is unsafe   // storing and comparing against same constants ok.
 #pragma clang diagnostic ignored "-Wglobal-constructors"    // warning : declaration requires a global destructor           // similar to above, not sure what the exact difference is.
@@ -100,7 +100,7 @@ namespace IMGUI_STB_NAMESPACE
 #pragma warning (disable: 4456)                             // declaration of 'xx' hides previous local declaration
 #endif
 
-#ifdef __clang__
+#if defined(__clang__)
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wunused-function"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
@@ -108,7 +108,7 @@ namespace IMGUI_STB_NAMESPACE
 #pragma clang diagnostic ignored "-Wcast-qual"              // warning : cast from 'const xxxx *' to 'xxx *' drops const qualifier //
 #endif
 
-#ifdef __GNUC__
+#if defined(__GNUC__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wtype-limits"              // warning: comparison is always true due to limited range of data type [-Wtype-limits]
 #pragma GCC diagnostic ignored "-Wcast-qual"                // warning: cast from type 'const xxxx *' to type 'xxxx *' casts away qualifiers
@@ -151,15 +151,15 @@ namespace IMGUI_STB_NAMESPACE
 #endif
 #endif
 
-#ifdef __GNUC__
+#if defined(__GNUC__)
 #pragma GCC diagnostic pop
 #endif
 
-#ifdef __clang__
+#if defined(__clang__)
 #pragma clang diagnostic pop
 #endif
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER)
 #pragma warning (pop)
 #endif