Build: make dependency on X-Lib explicit

The XV-Viewer widget in our GUI uses four direct calls
to the X-Lib. This was discovered by strict dependency checking,
as mandated by new Debian policy

admin/scons/Platform.py

@@ -129,6 +129,7 @@ def configure(env):
         problems.append('Xlib.h and Xutil.h required. Please install libx11-dev.')
     
     if not conf.CheckPkgConfig('xv')  : problems.append('Need libXv...')
+    if not conf.CheckPkgConfig('x11') : problems.append('Need X-lib...')   # for the xvdisplayer widget
     if not conf.CheckPkgConfig('xext'): problems.append('Need libXext.')
     
     

doc/technical/build/Dependencies.txt

@@ -90,7 +90,7 @@ Libraries
  - libgtkmm-2.4-dev
  - libcairomm-1.0-dev
  - libglibmm-2.4-dev, requiring glib2.0 and gthread-2.0
- - libxv-dev
+ - libxv-dev and X-lib footnote:[for the XV viewer widget `gui/output/xvdisplayer.cpp`]
  - librsvg-2.0 and librsvg2-dev for rendering Icons
  - libgdl-1-dev -- old version of the Gnome Docking Library footnote:[GDL isn't directly related to GNOME any more.
    We contributed to the improvement of this library in the past. These improvements went upstream