libitm: Work around assembler missing AVX insns.

From-SVN: r181246
diff --git a/libitm/configure.ac b/libitm/configure.ac
index c40ecb5..7de5cbe 100644
--- a/libitm/configure.ac
+++ b/libitm/configure.ac
@@ -237,6 +237,7 @@
 # had a chance to set XCFLAGS.
 LIBITM_CHECK_SYNC_BUILTINS
 LIBITM_CHECK_64BIT_SYNC_BUILTINS
+LIBITM_CHECK_AS_AVX
 
 # Cleanup and exit.
 CFLAGS="$save_CFLAGS"