[gtk+] Some updates to the installation instructions

[Matthias Clasen <matthiasc src gnome org>]

commit 65bb4b6dbe390e0c1d02bbe2fb4ccd11fc401893
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Mar 26 09:17:57 2012 -0400

    Some updates to the installation instructions
    
    Mention X11 build requirements (since XInput2 is something of
    an actual requirement, as opposed to 'newer than R5').

 INSTALL.in |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/INSTALL.in b/INSTALL.in
index 4b59bce..c18b995 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -9,6 +9,10 @@ GTK+ requires the following packages:
    GdkPixbuf @GDK_PIXBUF_REQUIRED_VERSION@, ATK @ATK_REQUIRED_VERSION@
    and cairo @CAIRO_REQUIRED_VERSION  
 
+ - Each GDK backend has its own backend-specific requirements. For
+   the X11 backend, X11 R6 and XInput version 2 (as well as a number
+   of other extensions) are required.
+
  - gobject-introspection @INTROSPECTION_REQUIRED_VERSION@ or newer.
 
 Simple install procedure