Make src/xrectsel a submodule

Add --enable-xrectsel option to configure, set by default to "check",
i.e. include the xrectsel subpackage if the xrectsel source is present.
This is helpful for quick build/test/install from Git, but packagers
are advised to package xrectsel separately.
Release 2.2.0 "in(ter)dependence"