diff --git a/ares/configure.ac b/ares/configure.ac
index d90d70b0c5f941d4f25a841d7ad002436462b5b9..12b61514fe56893d177cc2ccbed2e58b699e8467 100644
--- a/ares/configure.ac
+++ b/ares/configure.ac
@@ -162,7 +162,9 @@ esac
 
 AC_MSG_CHECKING([whether we are using the IBM C compiler])
 CURL_CHECK_DEF([__IBMC__], [], [silent])
-if test "$curl_cv_have_def___IBMC__" = "yes"; then
+CURL_CHECK_DEF([__IBMCPP__], [], [silent])
+if test "$curl_cv_have_def___IBMC__" = "yes" ||
+  test "$curl_cv_have_def___IBMCPP__" = "yes"; then
   AC_MSG_RESULT([yes])
   dnl Ensure that compiler optimizations are always thread-safe.
   CFLAGS="$CFLAGS -qthreaded"
diff --git a/configure.ac b/configure.ac
index ad25bde6e10c467772335c9ffdb4f03a258ce749..09fccedfea9373a8fade2d451d9e88ab6c56c947 100644
--- a/configure.ac
+++ b/configure.ac
@@ -251,7 +251,9 @@ esac
 
 AC_MSG_CHECKING([whether we are using the IBM C compiler])
 CURL_CHECK_DEF([__IBMC__], [], [silent])
-if test "$curl_cv_have_def___IBMC__" = "yes"; then
+CURL_CHECK_DEF([__IBMCPP__], [], [silent])
+if test "$curl_cv_have_def___IBMC__" = "yes" ||
+  test "$curl_cv_have_def___IBMCPP__" = "yes"; then
   AC_MSG_RESULT([yes])
   dnl Ensure that compiler optimizations are always thread-safe.
   CFLAGS="$CFLAGS -qthreaded"