fix up detection code for compiler defaulting to c++98

- remove the unconditional CONFIG+=c++11 from default_pre.prf, so the
  change actually has any effect
- fix up the description fields somewhat

this amends 091df96fb8, which was rushed in without maintainer review.

Change-Id: I88b859d6e2f0fe3d6a4771afe464a7c445e7b47b
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

configure.json

@@ -823,7 +823,7 @@
             "test": "common/c++1z"
         },
         "cxx98default": {
-            "description": "compilers defaulting to C++98",
+            "description": "compiler defaulting to C++98",
             "type": "compile",
             "test": "common/c++98default"
         },
@@ -1260,7 +1260,7 @@
             "output": [ "publicConfig", "privateConfig" ]
         },
         "cxx98default": {
-            "description": "Compilers defaulting to C++98",
+            "description": "Compiler defaults to C++98",
             "condition": "tests.cxx98default",
             "output": [ { "type": "publicConfig", "name": "c++11" } ]
         },

mkspecs/features/default_pre.prf

@@ -8,8 +8,6 @@ CONFIG = \
     testcase_targets import_plugins import_qpa_plugin \
     $$CONFIG
 
-CONFIG += c++11
-
 !build_pass:defined(QT_EDITION, var):!equals(QT_EDITION, "OpenSource"):!equals(QT_EDITION, "Preview") {
     #
     # call license checker (but cache result for one day)