Add XIPH_BSWAP32 configure macro to detect __builtin_bswap32() intrinsic.
diff --git a/configure.ac b/configure.ac
index af1593b..549024a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -66,6 +66,8 @@
 AC_CHECK_HEADERS(byteswap.h)
 AC_SUBST(HAVE_BYTESWAP_H)
 
+XIPH_C_BSWAP32
+
 XIPH_C_FIND_ENDIAN
 AC_DEFINE_UNQUOTED(CPU_IS_BIG_ENDIAN, ${ac_cv_c_big_endian},
 					[Target processor is big endian.])