fix and improve handling of BUILD_QT5 option

CMakeLists.txt

@@ -184,7 +184,6 @@ MESSAGE(STATUS "cmake: ${CMAKE_VERSION}")
 # ==============================================================================
 
 # Switch to build FreeCAD with Qt5
-OPTION(BUILD_QT5 "Build with Qt5." OFF)
 OPTION(BUILD_GUI "Build FreeCAD Gui. Otherwise you have only the command line and the Python import module." ON)
 OPTION(FREECAD_USE_EXTERNAL_ZIPIOS "Use system installed zipios++ instead of the bundled." OFF)
 OPTION(FREECAD_USE_EXTERNAL_SMESH "Use system installed smesh instead of the bundled." OFF)
@@ -218,6 +217,11 @@ if(MSVC)
         ENDIF()
     ELSE()
         set(FREECAD_LIBPACK_DIR ${CMAKE_SOURCE_DIR} CACHE PATH  "Directory of the FreeCAD LibPack")
+        IF(EXISTS ${FREECAD_LIBPACK_DIR}/lib/Qt5Core.lib)
+            OPTION(BUILD_QT5 "Build with Qt5." ON)
+        ELSE()
+            OPTION(BUILD_QT5 "Build with Qt5." OFF)
+        ENDIF()
     ENDIF()
 
     SET(LIBPACK_FOUND OFF)
@@ -236,15 +240,12 @@ if(MSVC)
             SET(COPY_LIBPACK_BIN_TO_BUILD ON)
             OPTION(FREECAD_COPY_LIBPACK_BIN_TO_BUILD "Copy larger libpack dependency 'bin' folder to the build direcctory." OFF)
         ENDIF()
-        # Default Qt5 ON if present in libpack
-        IF(EXISTS ${CMAKE_BINARY_DIR}/bin/Qt5Core.dll)
-            OPTION(BUILD_QT5 "Build with Qt5." OFF)
-        ENDIF()
     ELSE()
         MESSAGE("Libpack NOT found.\nIf you intend to use a Windows libpack, set the FREECAD_LIBPACK_DIR to the libpack directory.")
         MESSAGE(STATUS "Visit: https://github.com/apeltauer/FreeCAD/releases/ for Windows libpack downloads.")
     ENDIF()
 ELSE(MSVC)
+    OPTION(BUILD_QT5 "Build with Qt5." ON)
     OPTION(FREECAD_LIBPACK_USE "Use the LibPack to Build FreeCAD (only Win32 so far)." OFF)
     SET(FREECAD_LIBPACK_DIR ""  CACHE PATH  "Directory of the FreeCAD LibPack")
 ENDIF(MSVC)