Tentative GCC/Clang warning fixes with unused functions (cf #296)

imgui_demo.cpp

@@ -15,7 +15,6 @@
 #include <stdio.h>              // vsnprintf, sscanf, printf
 
 #ifdef _MSC_VER
-#pragma warning (disable: 4505) // unreferenced local function has been removed (stb stuff)
 #pragma warning (disable: 4996) // 'This function or variable may be unsafe': strcpy, strdup, sprintf, vsnprintf, sscanf, fopen
 #define snprintf _snprintf
 #endif

imgui_draw.cpp

@@ -31,6 +31,9 @@
 #pragma warning (disable: 4996) // 'This function or variable may be unsafe': strcpy, strdup, sprintf, vsnprintf, sscanf, fopen
 #define snprintf _snprintf
 #endif
+#ifdef __GNUC__
+#pragma GCC diagnostic ignored "-Wunused-function"          // warning: 'xxxx' defined but not used
+#endif
 
 //-------------------------------------------------------------------------
 // STB libraries implementation