Windows build fails if on different drives

Traceback (most recent call last):
  File "S:/qt_5.7.0_windows/qtwebengine/tools/buildscripts/gyp_qtwebengine", line 157, in <module>
    args.extend(['-Goutput_dir='+ os.path.relpath(output_dir, qtwebengine_root)])
  File "C:\Python27\lib\ntpath.py", line 529, in relpath
    % (path_prefix, start_prefix))
ValueError: path is on drive Q:, start on drive s:
Project MESSAGE: Running gyp_qtwebengine "Q:/tmp/windows/debug/qtwebengine/src/core" -D qt_cross_compile=0 -D qt_os="win32" -I config/windows.gypi -D qtwe_chromium_obj_dir="Q:/tmp/windows/debug/qtwebengine/src/core/Debug/obj/src/3rdparty/chromium" -D perl_exe="perl.exe" -D bison_exe="bison.exe" -D gperf_exe="gperf.exe" --no-parallel -D qt_gl="opengl" -G msvs_version=2015 -D use_qt=1 -D v8_use_external_startup_data=0 -D enable_basic_printing=1 -D enable_print_preview=0 -D enable_web_speech=0 -D enable_extensions=0 -D disable_nacl=1 -D remoting=0 -D use_ash=0 -D enable_widevine=1 -D fastbuild=2 -D qtwe_process_name_debug=QtWebEngineProcessd -D qtwe_process_name_release=QtWebEngineProcess -D disable_glibcxx_debug=1 -D remove_webcore_debug_symbols=1 -D remove_v8base_debug_symbols=1 -D disable_fatal_linker_warnings=1 -D sysroot="" -D target_arch=ia32 -D enable_spellcheck=0...
Project ERROR: -- running gyp_qtwebengine failed --