Fix IBM C and DEC/Compaq C compiler detection

configure.ac

@@ -213,9 +213,7 @@ esac
 
 AC_MSG_CHECKING([whether we are using the IBM C compiler])
 CURL_CHECK_DEF([__IBMC__], [], [silent])
-CURL_CHECK_DEF([__IBMCPP__], [], [silent])
-if test "$curl_cv_have_def___IBMC__" = "yes" ||
-  test "$curl_cv_have_def___IBMCPP__" = "yes"; then
+if test "$curl_cv_have_def___IBMC__" = "yes"; then
   AC_MSG_RESULT([yes])
   dnl Ensure that compiler optimizations are always thread-safe.
   CFLAGS="$CFLAGS -qthreaded"
@@ -234,7 +232,7 @@ fi
 AC_MSG_CHECKING([whether we are using the DEC/Compaq C compiler])
 CURL_CHECK_DEF([__DECC], [], [silent])
 CURL_CHECK_DEF([__DECC_VER], [], [silent])
-if test "$curl_cv_have_def___DECC" = "yes" ||
+if test "$curl_cv_have_def___DECC" = "yes" &&
   test "$curl_cv_have_def___DECC_VER" = "yes"; then
   AC_MSG_RESULT([yes])
   dnl Select strict ANSI C compiler mode