scons: Always build softpipe and llvmpipe (when llvm available).

These are our reference software rasterizers. They can build everywhere
and are a precious debugging tool.

Making them always present immensily simplifies the scons logic.

If people want to avoid building it is still possible to pass
direcotries and target names to scons to narrow the build.
diff --git a/SConstruct b/SConstruct
index ebced16..fdcd72b 100644
--- a/SConstruct
+++ b/SConstruct
@@ -102,11 +102,15 @@
 #######################################################################
 # Environment setup
 
-# Always build trace and identity drivers
+# Always build trace, identity, softpipe, and llvmpipe (where possible)
 if 'trace' not in env['drivers']:
     env['drivers'].append('trace')
 if 'identity' not in env['drivers']:
     env['drivers'].append('identity')
+if 'softpipe' not in env['drivers']:
+    env['drivers'].append('softpipe')
+if env['llvm'] and 'llvmpipe' not in env['drivers']:
+    env['drivers'].append('llvmpipe')
 
 # Includes
 env.Append(CPPPATH = [