cmake: fix detection of libXext and libXv

Also require them only if appropriate backends are getting built

Signed-off-by: Alexander Golubev <fatzer2@gmail.com>
(cherry picked from commit 433b3113a3)

ConfigureChecks.cmake

@@ -31,13 +31,14 @@ endif( WITH_GCC_VISIBILITY )
 find_package( X11 )
 
 
-##### check for XShm, Xv
-
-if( X11_FOUND )
-    elseif( NOT X11_XShm_FOUND )
-            tde_message_fatal( "libXext is required, but was not found on your system." )
-    elseif( NOT X11_Xv_FOUND )
-            tde_message_fatal( "libXv is required, but was not found on your system." )
+##### check for XExt, Xv
+
+if( NOT X11_FOUND )
+    tde_message_fatal( "X11 is required, but was not found on your system." )
+elseif( NOT X11_XExt_FOUND AND ( BUILD_KXVPLAYER OR BUILD_KXINEPLAYER ) )
+    tde_message_fatal( "libXext is required, but was not found on your system." )
+elseif( NOT X11_Xv_FOUND AND BUILD_KXVPLAYER )
+    tde_message_fatal( "libXv is required, but was not found on your system." )
 endif()