stb_image_write: Make globals extern "C" in C++

Fixes issue #921

stb_image_write.h

@@ -167,9 +167,9 @@ LICENSE
 #endif
 
 #ifndef STB_IMAGE_WRITE_STATIC  // C++ forbids static forward declarations
-extern int stbi_write_tga_with_rle;
-extern int stbi_write_png_compression_level;
-extern int stbi_write_force_png_filter;
+STBIWDEF int stbi_write_tga_with_rle;
+STBIWDEF int stbi_write_png_compression_level;
+STBIWDEF int stbi_write_force_png_filter;
 #endif
 
 #ifndef STBI_WRITE_NO_STDIO