scons: Make LLVM a black-white dependency.

Now that draw depends on llvm it is very difficult to correctly handle
broken llvm installations. Either the user requests LLVM and it needs to
supply a working installation. Or it doesn't, and it gets no LLVM
accelerate pipe drivers.

scons/gallium.py

@@ -142,8 +142,6 @@ def generate(env):
     # configuration. See also http://www.scons.org/wiki/AdvancedBuildExample
     build_topdir = 'build'
     build_subdir = env['platform']
-    if env['llvm']:
-        build_subdir += "-llvm"
     if env['machine'] != 'generic':
         build_subdir += '-' + env['machine']
     if env['debug']:
@@ -471,6 +469,10 @@ def generate(env):
     # Default libs
     env.Append(LIBS = [])
 
+    # Load LLVM
+    if env['llvm']:
+        env.Tool('llvm')
+
     # Custom builders and methods
     env.Tool('custom')
     createInstallMethods(env)