The Yocto-based GNOME 3.20 SDK used to build the LO Flatpak has a broken
xml2-config.  I cannot understand why the previous workaround worked fine with
raptor2-2.0.9 in LO 5.2 and fails with raptor2-2.0.15 since LO 5.3, but this
updated workaround keeps raptor's configure happy.

Change-Id: Ibfb2cb8a718f744e1bb4045082520fb186d6062b