qtserialport requires udev unconditionally

The current version of qtserialport requires udev unconditionally.

It seems, the variable QMAKE_LIBS_LIBUDEV (qtcore) is getting set to a library path if qtgui is not present and to "-ludev" if it is.

Later, "qtConfig(libudev)"  src/serialport/serialport-lib.pri would pick up its value.

As a result, qtserialport fails to build if qtgui is not installed or compiled without udev.

See more details at Gentoo bugzilla:
https://bugs.gentoo.org/673532

Please add udev conditional flag, similar with qtcore or find a better solution
Thank you.