CMake: Allow no install + custom on-device prefix for desktop builds

Allow such a combination
- staging prefix (CMAKE_STAGING_PREFIX / -extprefix) set to the
  qtbase build dir
- install prefix (CMAKE_INSTALL_PREFIX / -prefix / on-device prefix) set
  to some custom location
even for non-cross builds.

An example would be
    configure -prefix    /usr \
              -extprefix ~/qt/qtbase_build_dir

CMake will put the Qt libraries in the qtbase build dir, ninja install
will not be required, but ultimately in order to run applications,
the Qt libraries are expected to be in /usr.

Support for this scenario was originally added for cross-builds in
062318f , but not desktop builds.

Such a build is useful when you want to have install rpaths different
from where Qt is initially installed to (the staging prefix).

This case doesn't really happen often when targeting desktop
platforms, it's mostly used for cross-compilation (e.g yocto).

Being able to have the same setup with a desktop build is nevertheless
useful for faster iteration on build system issues in such a scenario.

Amends 062318f

Pick-to: 6.2 6.3
Change-Id: I42be3628a30025f14eebaf0a79401b54e95cde26
Reviewed-by: Qt CI Bot <[email protected]>
Reviewed-by: Jörg Bornemann <[email protected]>

cmake/QtBuild.cmake

@@ -140,7 +140,7 @@ qt_configure_process_path(INSTALL_DESCRIPTIONSDIR
                          "${INSTALL_DATADIR}/modules"
                           "Module description files directory")
 
-if(CMAKE_CROSSCOMPILING AND NOT "${CMAKE_STAGING_PREFIX}" STREQUAL "")
+if(NOT "${CMAKE_STAGING_PREFIX}" STREQUAL "")
     set(QT_STAGING_PREFIX "${CMAKE_STAGING_PREFIX}")
 else()
     set(QT_STAGING_PREFIX "${CMAKE_INSTALL_PREFIX}")

cmake/QtBuildInternals/QtBuildInternalsConfig.cmake

@@ -666,15 +666,21 @@ function(qt_compute_relative_path_from_cmake_config_dir_to_prefix)
     #
     # Similar code exists in Qt6CoreConfigExtras.cmake.in and src/corelib/CMakeLists.txt which
     # might not be needed anymore.
+    if(CMAKE_STAGING_PREFIX)
+        set(__qt_prefix "${CMAKE_STAGING_PREFIX}")
+    else()
+        set(__qt_prefix "${CMAKE_INSTALL_PREFIX}")
+    endif()
+
     if(QT_WILL_INSTALL)
         get_filename_component(clean_config_prefix
-                               "${CMAKE_INSTALL_PREFIX}/${QT_CONFIG_INSTALL_DIR}" ABSOLUTE)
+                               "${__qt_prefix}/${QT_CONFIG_INSTALL_DIR}" ABSOLUTE)
     else()
         get_filename_component(clean_config_prefix "${QT_CONFIG_BUILD_DIR}" ABSOLUTE)
     endif()
     file(RELATIVE_PATH
          qt_path_from_cmake_config_dir_to_prefix
-         "${clean_config_prefix}" "${CMAKE_INSTALL_PREFIX}")
+         "${clean_config_prefix}" "${__qt_prefix}")
      set(qt_path_from_cmake_config_dir_to_prefix "${qt_path_from_cmake_config_dir_to_prefix}"
          PARENT_SCOPE)
 endfunction()

cmake/QtSetup.cmake

@@ -137,12 +137,9 @@ if (PROJECT_NAME STREQUAL "QtBase" AND NOT QT_BUILD_STANDALONE_TESTS)
             "Install path prefix, prepended onto install directories." FORCE)
         unset(__qt_default_prefix)
     endif()
-    if(CMAKE_CROSSCOMPILING)
+    if(CMAKE_STAGING_PREFIX)
         set(__qt_prefix "${CMAKE_STAGING_PREFIX}")
     else()
-        set(__qt_prefix "")
-    endif()
-    if(__qt_prefix STREQUAL "")
         set(__qt_prefix "${CMAKE_INSTALL_PREFIX}")
     endif()
     if(__qt_prefix STREQUAL QtBase_BINARY_DIR)