In order to build SDL2_image, I had to edit IMG_png.c and change:
/* Check for the older version of libpng */
#if (PNG_LIBPNG_VER_MAJOR == 1)
#if (PNG_LIBPNG_VER_MINOR < 4)
...to:
/* Check for the older version of libpng */
#if (PNG_LIBPNG_VER_MAJOR == 1)
#if (PNG_LIBPNG_VER_MINOR < 5)...