Try to fix windows build

external/CMakeLists.txt

@@ -1077,7 +1077,7 @@ if (WIN32 OR APPLE)
                 CMAKE_ARGS ${COMMON_CMAKE_ARGS}
                 -DINSTALL_UDEV_RULES=${RTLSDR_UDEV}
                 -DLIBUSB_LIBRARIES=${LIBUSB_LIBRARIES}
-                -DCMAKE_C_FLAGS=-I${RTLSDR_LIBUSB_INCLUDE}
+                -DLIBUSB_INCLUDE_DIRS=${RTLSDR_LIBUSB_INCLUDE}
                 -DTHREADS_PTHREADS_INCLUDE_DIR=${PTHREADS4W_INCLUDE_DIR}
                 -DTHREADS_PTHREADS_WIN32_LIBRARY=${PTHREADS4W_LIBRARIES}
                 BUILD_BYPRODUCTS "${LIBRTLSDR_LIBRARIES}"