On a minimal QT installation, widgets may not necessarily be
added and it needs to be explicitely done. Modifying ./configure
directly given tarball lacks some files to run eautoreconf.

https://bugs.gentoo.org/831361
--- a/configure
+++ b/configure
@@ -25748,2 +25748,3 @@
 qtHaveModule(testlib):           QT += testlib
+qtHaveModule(widgets):           QT += widgets
 percent.target = %