I just tried to compile digikam master and while it works fine on QT5.11.1, it fails on QT5.7.1 (that's just what is provided on debian testing/stable respectively).

digikam/core/utilities/searchwindow/searchtabheader.cpp: In member function ‘virtual void Digikam::KeywordLineEdit::contextMenuEvent(QContextMenuEvent*)’:
digikam/core/utilities/searchwindow/searchtabheader.cpp:119:21: error: invalid use of incomplete type ‘class QContextMenuEvent’
         menu->exec(e->globalPos());
                     ^~
In file included from /usr/include/x86_64-linux-gnu/qt5/QtWidgets/QWidget:1:0,
                 from digikam/core/utilities/searchwindow/searchtabheader.h:29,
                 from digikam/core/utilities/searchwindow/searchtabheader.cpp:24:
/usr/include/x86_64-linux-gnu/qt5/QtWidgets/qwidget.h:80:7: note: forward declaration of ‘class QContextMenuEvent’
 class QContextMenuEvent;
       ^~~~~~~~~~~~~~~~~

Digikam depends on >=5.6.0 (https://www.digikam.org/api/index.html#externaldeps). I understand that's really old, but the fix is trivial, so why not have it:

--- a/core/utilities/searchwindow/searchtabheader.cpp
+++ b/core/utilities/searchwindow/searchtabheader.cpp
@@ -39,6 +39,7 @@
 #include <QInputDialog>
 #include <QIcon>
 #include <QMenu>
+#include <QContextMenuEvent>
 
 // KDE includes

Any objections?