configs: Fix linking with static libGL and --as-needed

Linking of the programs breaks when using a static libGL and the GNU ld
option --as-needed. This is because libXext is needed for the XShm
functions.
diff --git a/configs/linux-static b/configs/linux-static
index 2a77d4c..1ee16be 100644
--- a/configs/linux-static
+++ b/configs/linux-static
@@ -22,4 +22,5 @@
 GLW_LIB_DEPS = 
 
 # Need to specify all libraries we may need
-APP_LIB_DEPS = $(EXTRA_LIB_PATH) -lX11 -lXmu -lXt -lXi -lpthread -lstdc++ -lm
+APP_LIB_DEPS = $(EXTRA_LIB_PATH) -lX11 -lXext -lXmu -lXt -lXi -lpthread \
+	-lstdc++ -lm