Gentoo still ships QtWebKit in 5.6 and later, so don't try and fail to use QtWebEngine.

--- a/src/src.pro
+++ b/src/src.pro
@@ -113,21 +113,11 @@
 }
 
 greaterThan(QT_MAJOR_VERSION, 4) {
-    # Qt 5
-    greaterThan(QT_MINOR_VERSION, 5) {
-        # Qt 5.6+
-        error("You use Qt5.6+ - QWebEngine is not yet suitable for kchmviewer and is not supported")
-        QT += webengine webenginewidgets
-        DEFINES += USE_WEBENGINE
-        SOURCES += viewwindow_webengine.cpp dataprovider_qwebengine.cpp
-        HEADERS += dataprovider_qwebengine.h viewwindow_webengine.h
-    } else {
         # Qt 5.0-5.5
         QT += webkit webkitwidgets
         DEFINES += USE_WEBKIT
         SOURCES += viewwindow_webkit.cpp dataprovider_qwebkit.cpp
         HEADERS += dataprovider_qwebkit.h viewwindow_webkit.h
-    }
 } else {
     message("Qt4 is not supported anymore, please do not report any errors")
     QT += webkit webkitwidgets