icu config test: require C++11 (actually C++17)

ICU headers now require C++17, and while the Portfile already sets
`QMAKE_CXXFLAGS_GNUCXX11=-std=c++17`, the config test does not actually add
`-std=${QMAKE_CXXFLAGS_GNUCXX11}` unless the project file declares that C++11
is required.

--- ./src/corelib/configure.json.orig   2025-09-18 11:39:50
+++ ./src/corelib/configure.json        2025-09-18 11:40:27
@@ -102,7 +102,8 @@
                     "UCollator *collator = ucol_open(\"ru_RU\", &status);",
                     "if (!U_FAILURE(status))",
                     "    ucol_close(collator);"
-                ]
+                ],
+                "qmake": "CONFIG += c++11"
             },
             "headers": [ "unicode/utypes.h", "unicode/ucol.h", "unicode/ustring.h" ],
             "sources": [