GL screensavers fail to load Mesa renderer add-on

The screensaver add-ons are properly linked against libGL. The libGL
code tries to load the swrast add-on. But this fails because the
BGLRenderer constructor (which is provided by libGL) is not found.

It seems that when loading an add-on, libraries linked to by other
add-ons (and not the app itself) are not searched to resolve symbols. To
avoid this issue, we now link ScreenSaver and screen_blanker to libGL,
so the GL renderer add-on can find it.

Fixes #10206

src/bin/screen_blanker/Jamfile

@@ -17,6 +17,7 @@ BinCommand screen_blanker :
 		# link against libscreensaver.so
 
 	: be game localestub libscreensaver.so [ TargetLibsupc++ ]
+	[ BuildFeatureAttribute mesa : library ]
 	: screen_blanker.rdef
 ;
 

src/preferences/screensaver/Jamfile

@@ -19,6 +19,7 @@ Preference ScreenSaver :
 		# needed for some old/broken screen saver add-ons that do not
 		# link against libscreensaver.so
 	: be game libscreensaver.so localestub tracker [ TargetLibstdc++ ]
+	[ BuildFeatureAttribute mesa : library ]
 	: ScreenSaver.rdef
 	;