configure.ac: Use AX_GCC_BUILTIN to check availability of __builtin_bswap32 v2

v2:
  - Remove unnecessary AC_SUBST

Reviewed-by: Matt Turner <mattst88@gmail.com>
diff --git a/configure.ac b/configure.ac
index 24ffc54..4cfdedb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -119,6 +119,10 @@
         GEN_ASM_OFFSETS=yes
     fi
 fi
+
+dnl Check for compiler builtins
+AX_GCC_BUILTIN([__builtin_bswap32])
+
 AM_CONDITIONAL([GEN_ASM_OFFSETS], test "x$GEN_ASM_OFFSETS" = xyes)
 
 dnl Make sure the pkg-config macros are defined