diff --git a/cmake/QtBuild.cmake b/cmake/QtBuild.cmake
index 2b88456878f..e04069c6487 100644
--- a/cmake/QtBuild.cmake
+++ b/cmake/QtBuild.cmake
@@ -372,30 +372,39 @@ else()
     set(QT_QMAKE_HOST_MKSPEC "${QT_QMAKE_TARGET_MKSPEC}")
 endif()
 
-# Platform definition dir provided by user on command line.
-# Derive the absolute one relative to the current source dir.
-if(QT_PLATFORM_DEFINITION_DIR)
-    set(QT_DEFAULT_PLATFORM_DEFINITION_DIR "${QT_PLATFORM_DEFINITION_DIR}")
-    get_filename_component(
-        QT_DEFAULT_PLATFORM_DEFINITION_DIR_ABSOLUTE
-        "${QT_PLATFORM_DEFINITION_DIR}"
-        ABSOLUTE)
-elseif(QT_QMAKE_TARGET_MKSPEC)
-    # Used by consumers of prefix builds via INSTALL_INTERFACE (relative path).
-    set(QT_DEFAULT_PLATFORM_DEFINITION_DIR "${INSTALL_MKSPECSDIR}/${QT_QMAKE_TARGET_MKSPEC}")
-    # Used by qtbase itself and consumers of non-prefix builds via BUILD_INTERFACE (absolute path).
-    set(QT_DEFAULT_PLATFORM_DEFINITION_DIR_ABSOLUTE "${QT_MKSPECS_DIR}/${QT_QMAKE_TARGET_MKSPEC}")
+# Used by consumers of prefix builds via INSTALL_INTERFACE (relative path).
+set(QT_DEFAULT_PLATFORM_DEFINITION_DIR "${INSTALL_MKSPECSDIR}/${QT_QMAKE_TARGET_MKSPEC}")
 
-    if(NOT EXISTS "${QT_DEFAULT_PLATFORM_DEFINITION_DIR_ABSOLUTE}")
-        file(GLOB known_platforms
-            LIST_DIRECTORIES true
-            RELATIVE "${QT_MKSPECS_DIR}"
-            "${QT_MKSPECS_DIR}/*"
-        )
-        list(JOIN known_platforms "\n    " known_platforms)
-        message(FATAL_ERROR "Unknown platform ${QT_QMAKE_TARGET_MKSPEC}\n\
+# Used by qtbase in prefix builds via BUILD_INTERFACE
+set(QT_PLATFORM_DEFINITION_BUILD_INTERFACE_BASE_DIR
+    "${CMAKE_CURRENT_LIST_DIR}/../mkspecs/"
+)
+
+# Used by qtbase and consumers in non-prefix builds via BUILD_INTERFACE
+if(NOT QT_WILL_INSTALL)
+    set(QT_PLATFORM_DEFINITION_BUILD_INTERFACE_BASE_DIR
+        "${CMAKE_BINARY_DIR}/${INSTALL_MKSPECSDIR}"
+    )
+endif()
+
+get_filename_component(QT_PLATFORM_DEFINITION_BUILD_INTERFACE_DIR
+    "${QT_PLATFORM_DEFINITION_BUILD_INTERFACE_BASE_DIR}/${QT_QMAKE_TARGET_MKSPEC}"
+    ABSOLUTE
+)
+set(QT_PLATFORM_DEFINITION_BUILD_INTERFACE_DIR
+    "${QT_PLATFORM_DEFINITION_BUILD_INTERFACE_DIR}"
+    CACHE INTERNAL "Path to directory that contains qplatformdefs.h"
+)
+
+if(NOT EXISTS "${QT_MKSPECS_DIR}/${QT_QMAKE_TARGET_MKSPEC}")
+    file(GLOB known_platforms
+        LIST_DIRECTORIES true
+        RELATIVE "${QT_MKSPECS_DIR}"
+        "${QT_MKSPECS_DIR}/*"
+    )
+    list(JOIN known_platforms "\n    " known_platforms)
+    message(FATAL_ERROR "Unknown platform ${QT_QMAKE_TARGET_MKSPEC}\n\
 Known platforms:\n    ${known_platforms}")
-    endif()
 endif()
 
 if(NOT DEFINED QT_DEFAULT_PLATFORM_DEFINITIONS)
@@ -406,8 +415,7 @@ set(QT_PLATFORM_DEFINITIONS ${QT_DEFAULT_PLATFORM_DEFINITIONS}
     CACHE STRING "Qt platform specific pre-processor defines")
 set(QT_PLATFORM_DEFINITION_DIR "${QT_DEFAULT_PLATFORM_DEFINITION_DIR}"
     CACHE PATH "Path to directory that contains qplatformdefs.h")
-set(QT_PLATFORM_DEFINITION_DIR_ABSOLUTE "${QT_DEFAULT_PLATFORM_DEFINITION_DIR_ABSOLUTE}"
-    CACHE INTERNAL "Path to directory that contains qplatformdefs.h")
+
 set(QT_NAMESPACE "" CACHE STRING "Qt Namespace")
 
 include(QtGlobalStateHelpers)
diff --git a/cmake/QtPlatformTargetHelpers.cmake b/cmake/QtPlatformTargetHelpers.cmake
index 0ff8761f33b..b98894829f5 100644
--- a/cmake/QtPlatformTargetHelpers.cmake
+++ b/cmake/QtPlatformTargetHelpers.cmake
@@ -7,7 +7,7 @@ function(qt_internal_setup_public_platform_target)
     add_library(${INSTALL_CMAKE_NAMESPACE}::Platform ALIAS Platform)
     target_include_directories(Platform
         INTERFACE
-        $<BUILD_INTERFACE:${QT_PLATFORM_DEFINITION_DIR_ABSOLUTE}>
+        $<BUILD_INTERFACE:${QT_PLATFORM_DEFINITION_BUILD_INTERFACE_DIR}>
         $<BUILD_INTERFACE:${PROJECT_BINARY_DIR}/include>
         $<INSTALL_INTERFACE:${QT_PLATFORM_DEFINITION_DIR}>
         $<INSTALL_INTERFACE:${INSTALL_INCLUDEDIR}>