make unspecified/empty HostPrefix fall back to Prefix

this is needed for backwards-compatibility, and generally makes sense.

Task-number: QTBUG-24839
Change-Id: I42b008a9dfb43cc2ebe49c8baa2c29f5d6a75897
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>

src/corelib/global/qlibraryinfo.cpp

@@ -295,6 +295,12 @@ QLibraryInfo::rawLocation(LibraryLocation loc)
 
             ret = config->value(key, defaultValue).toString();
 
+#ifdef QT_BUILD_QMAKE
+            if (ret.isEmpty() && loc == HostPrefixPath)
+                ret = config->value(QLatin1String(qtConfEntries[PrefixPath].key),
+                                    QLatin1String(qtConfEntries[PrefixPath].value)).toString();
+#endif
+
             // expand environment variables in the form $(ENVVAR)
             int rep;
             QRegExp reg_var(QLatin1String("\\$\\(.*\\)"));