https://bugs.gentoo.org/634662
Author of patch: Rick Harris 
--- a/3rdparty/bgfx/include/bgfx/bgfxplatform.h
+++ b/3rdparty/bgfx/include/bgfx/bgfxplatform.h
@@ -253,7 +253,7 @@
 
 #endif // BX_PLATFORM_
 
-#if defined(_SDL_syswm_h)
+#if defined(_SDL_syswm_h) || defined(SDL_syswm_h_)
 // If SDL_syswm.h is included before bgfxplatform.h we can enable SDL window
 // interop convenience code.