CMake: Temporarily disable building external examples in prefix builds

There are issues when building examples as external projects in the CI
in child repos like qtsvg.

QEMU configurations fail to find some CMake Config files, Windows
configs fail to find libraries.

Until these issues are fixed, build examples in-tree (without using
external projects) like we did before.

Temporarily disables 98c89c8

Pick-to: 6.2 6.3
Task-number: QTBUG-90820
Task-number: QTBUG-96232
Change-Id: Ia4b39812b27cfde5f5a103fd39cc1cb032842643
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

cmake/QtSetup.cmake

@@ -176,7 +176,15 @@ enable_testing()
 option(QT_BUILD_EXAMPLES "Build Qt examples" OFF)
 option(QT_BUILD_EXAMPLES_BY_DEFAULT "Should examples be built as part of the default 'all' target." ON)
 
-option(QT_BUILD_EXAMPLES_AS_EXTERNAL "Should examples be built as ExternalProjects." ON)
+# FIXME: Support prefix builds as well QTBUG-96232
+if(QT_WILL_INSTALL)
+    set(_qt_build_examples_as_external OFF)
+else()
+    set(_qt_build_examples_as_external ON)
+endif()
+option(QT_BUILD_EXAMPLES_AS_EXTERNAL "Should examples be built as ExternalProjects."
+       ${_qt_build_examples_as_external})
+unset(_qt_build_examples_as_external)
 
 option(QT_BUILD_MANUAL_TESTS "Build Qt manual tests" OFF)
 option(QT_BUILD_MINIMAL_STATIC_TESTS "Build minimal subset of tests for static Qt builds" OFF)