configure.ac: Improve portability of the code introduced in r15423

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15452 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/configure.ac b/configure.ac
index ff459ff..3024e3c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1350,7 +1350,7 @@
 AC_DEFUN([AC_HWCAP_CONTAINS_FLAG],[
   AUXV_CHECK_FOR=$1
   AC_MSG_CHECKING([if AT_HWCAP contains the $AUXV_CHECK_FOR indicator])
-  if `LD_SHOW_AUXV=1 /bin/true | grep ^AT_HWCAP | grep -q -w ${AUXV_CHECK_FOR}`
+  if LD_SHOW_AUXV=1 `which true` | grep ^AT_HWCAP | grep -q -w ${AUXV_CHECK_FOR}
   then
     AC_MSG_RESULT([yes])
     AC_SUBST([$2],[yes])