CMake: Fix no_prefix feature to take into account the staging prefix

Instead of complicating the condition even more, just use the value of
QT_WILL_INSTALL which is determined in QtSetup before configure.cmake
is loaded.

The AUTODETECT part is needed to ensure that
 -developer-build
implies -no-prefix.

The CONDITION part is needed so that
 -extprefix /tmp/sysroot -no-prefix
correctly errors out saying that this can't be a non-prefix build,
unless
 -extprefix ${qtbase_build_dir} -no-prefix
is passed.

Pick-to: 6.2 6.3
Change-Id: Ie4f5a91281bf2fbe1bd0744de05d57f43fe992e7
Reviewed-by: Jörg Bornemann <[email protected]>

configure.cmake

@@ -484,11 +484,8 @@ qt_feature("developer-build" PRIVATE
 )
 qt_feature("no-prefix" PRIVATE
     LABEL "No prefix build"
-    # The var expansion on the right hand side is on purpose
-    # because the custom condition evaluator only expands the lhs
-    CONDITION (CMAKE_INSTALL_PREFIX STREQUAL "${QtBase_BINARY_DIR}")
-              OR CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT
-    AUTODETECT QT_FEATURE_developer_build
+    AUTODETECT NOT QT_WILL_INSTALL
+    CONDITION NOT QT_WILL_INSTALL
 )
 qt_feature("private_tests" PRIVATE
     LABEL "Developer build: private_tests"