Ignore GCC 6 warnings (#883)

imgui_demo.cpp

@@ -38,7 +38,9 @@
 #pragma GCC diagnostic ignored "-Wformat-security"              // warning : format string is not a string literal (potentially insecure)
 #pragma GCC diagnostic ignored "-Wdouble-promotion"             // warning: implicit conversion from 'float' to 'double' when passing argument to function
 #pragma GCC diagnostic ignored "-Wconversion"                   // warning: conversion to 'xxxx' from 'xxxx' may alter its value
-//#pragma GCC diagnostic ignored "-Wmisleading-indentation"     // warning: this 'if' clause does not guard this statement      // GCC 6.0+ only. See #883 on github.
+#if (__GNUC__ >= 6)
+#pragma GCC diagnostic ignored "-Wmisleading-indentation"       // warning: this 'if' clause does not guard this statement      // GCC 6.0+ only. See #883 on github.
+#endif
 #endif
 
 // Play it nice with Windows users. Notepad in 2015 still doesn't display text data with Unix-style \n.