configure.ac: add AC_C_BIGENDIAN

this defines WORDS_BIGENDIAN, replacing uses of
__BIG_ENDIAN__/__BYTE_ORDER__ with it

+ fixes lossless BGRA output with big-endian toolchains
  that do not define __BIG_ENDIAN__ (codesourcery mips gcc)

Change-Id: Ieaccd623292d235343b5e34b7a720fc251c432d7

examples/tiffdec.c

@@ -97,7 +97,7 @@ int ReadTIFF(const char* const filename,
       pic->width = width;
       pic->height = height;
       // TIFF data is ABGR
-#ifdef __BIG_ENDIAN__
+#ifdef WORDS_BIGENDIAN
       TIFFSwabArrayOfLong(raster, width * height);
 #endif
       pic->use_argb = 1;