configure: improve warning when all qpa plugins disabled with features.gui

The mentioned README file has never been very trustworthy and
after 863c688749 it has become
very minimal - it doesn't contain anything that is not already
present in "configure --help" and enforced by the configure
process. Furthermore, the warning was XCB specific, but Qt
supports also other QPA plugins on linux.

Change-Id: I3211dda3f294cbcd5f3d15fe8c21a1af7627f048
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

config_help.txt

@@ -293,7 +293,7 @@ Gui, printing, widget options:
     -kms ............... Enable backends for KMS [auto] (Linux only)
     -linuxfb ........... Enable Linux Framebuffer support [auto] (Linux only)
     -mirclient ......... Enable Mir client support [no] (Linux only)
-    -xcb ............... Select used xcb-* libraries [system/qt/no]
+    -xcb ............... Enable X11 support. Select used xcb-* libraries [system/qt/no]
                          (-qt-xcb still uses system version of libxcb itself)
 
   Input backends:

src/gui/configure.json

@@ -1830,8 +1830,7 @@ or may depend on your system and XQuartz setup."
             "condition": "features.gui && config.linux && !config.android && !features.xcb && !features.eglfs && !features.directfb && !features.linuxfb && !features.mirclient",
             "message": "No QPA platform plugin enabled! This will
 produce a Qt that cannot run GUI applications.
-The dependencies needed for xcb to build are listed in
-src/plugins/platforms/xcb/README"
+See \"Platform backends\" in the output of --help."
         },
         {
             "type": "warning",