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.

common.py

@@ -3,6 +3,7 @@
 
 import os
 import os.path
+import subprocess
 import sys
 import platform as _platform
 
@@ -33,6 +34,11 @@ else:
 	default_machine = _platform.machine()
 default_machine = _machine_map.get(default_machine, 'generic')
 
+if 'LLVM' in os.environ or subprocess.call(['llvm-config', '--version'], stdout=subprocess.PIPE) == 0:
+    default_llvm = 'yes'
+else:
+    default_llvm = 'no'
+
 if default_platform in ('linux', 'freebsd'):
 	default_dri = 'yes'
 elif default_platform in ('winddk', 'windows', 'wince', 'darwin'):
@@ -61,5 +67,5 @@ def AddOptions(opts):
 	opts.Add(EnumOption('platform', 'target platform', default_platform,
 											 allowed_values=('linux', 'cell', 'windows', 'winddk', 'wince', 'darwin', 'embedded')))
 	opts.Add('toolchain', 'compiler toolchain', 'default')
-	opts.Add(BoolOption('llvm', 'use LLVM', 'no'))
+	opts.Add(BoolOption('llvm', 'use LLVM', default_llvm))
 	opts.Add(BoolOption('dri', 'build DRI drivers', default_dri))