scons: Fix xlib build.

After glsl rework merge.
diff --git a/src/glsl/SConscript b/src/glsl/SConscript
index 101487d..8e18626 100644
--- a/src/glsl/SConscript
+++ b/src/glsl/SConscript
@@ -23,7 +23,7 @@
     'cl/sl_cl_parse.c',
 ]
 
-glsl = env.StaticLibrary(
+glsl = env.ConvenienceLibrary(
     target = 'glsl',
     source = sources,
 )