Add user friendly warning to basic fontdb when there are no fonts

Qt 5.7 no longer contains lib/fonts. This is a regression for users
relying on Qt's own fonts when running on Embedded Linux systems without
fontconfig. To alleviate the pain, show a friendly message about this.
The old "is Qt installed correctly" note is out-of-date anyhow now.

Change-Id: Idcb732fb65ed9c45529a0bcc3168227e0969bea0
Reviewed-by: Louai Al-Khanji <louai.al-khanji@theqtcompany.com>

src/platformsupport/fontdatabases/basic/qbasicfontdatabase.cpp

@@ -64,8 +64,9 @@ void QBasicFontDatabase::populateFontDatabase()
     QDir dir(fontpath);
 
     if (!dir.exists()) {
-        qWarning("QFontDatabase: Cannot find font directory %s - is Qt installed correctly?",
-               qPrintable(fontpath));
+        qWarning("QFontDatabase: Cannot find font directory %s.\n"
+                 "Note that Qt no longer ships fonts. Deploy some (from http://dejavu-fonts.org for example) or switch to fontconfig.",
+                 qPrintable(fontpath));
         return;
     }