Add conditional #ifdef prior to imconfig.h to facilitate inclusion in build systems (#255)

imgui.h

@@ -6,7 +6,9 @@
 
 #pragma once
 
+#if !defined(IMGUI_DISABLE_INCLUDE_IMCONFIG_H) || defined(IMGUI_INCLUDE_IMCONFIG_H)
 #include "imconfig.h"       // User-editable configuration file
+#endif
 #include <float.h>          // FLT_MAX
 #include <stdarg.h>         // va_list
 #include <stddef.h>         // ptrdiff_t, NULL