check validity of the host Qt

When trying to configure a cross-build, it would be nice if we checked the validity of the host Qt.

This suggestion resulted from the following original build error, encountered by the reporter:

Trying to build 6.3 with wasm support. 

C:\Qt\6.3.0\wasm_32_asyncify>..\Src\configure.bat -qt-host-path C:\Qt\6.3.0\wasm_32 -no-warnings-are-errors -platform wasm-emscripten -device-option QT_EMSCRIPTEN_ASYNCIFY=1 -prefix C:\Qt\6.3.0\wasm_32_asyncify

Result:

Searching for tool 'Qt6::moc' in package Qt6CoreTools.

Could NOT find Qt6CoreTools (missing: Qt6CoreTools_DIR)
CMake Error at qtbase/cmake/QtToolHelpers.cmake:156 (message):
The tool "Qt6::moc" was not found in the Qt6CoreTools package. Package
found: 0
Call Stack (most recent call first):
qtbase/src/tools/moc/CMakeLists.txt:8 (qt_internal_add_tool)

Configuring incomplete, errors occurred!

See also "C:/Qt/6.3.0/wasm_32_asyncify/CMakeFiles/CMakeOutput.log".
See also "C:/Qt/6.3.0/wasm_32_asyncify/CMakeFiles/CMakeError.log".
CMake Error at C:/Qt/6.3.0/Src/qtbase/cmake/QtProcessConfigureArgs.cmake:957 (message):
CMake exited with code 1.