libgl-xlib: enable galahad support

If the GALLIUM_GALAHAD env var is 1 we'll wrap the regular driver with
the galahad validation driver.

src/gallium/targets/libgl-xlib/SConscript

@@ -48,6 +48,10 @@ if True:
     env.Append(CPPDEFINES = 'GALLIUM_SOFTPIPE')
     env.Prepend(LIBS = [softpipe])
 
+if True:
+    env.Append(CPPDEFINES = 'GALLIUM_GALAHAD')
+    env.Prepend(LIBS = [galahad])
+
 if env['llvm']:
     env.Append(CPPDEFINES = 'GALLIUM_LLVMPIPE')
     env.Tool('udis86')