scons: Recognize LLVM_CONFIG environment variable.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

common.py

@@ -59,7 +59,7 @@ if target_platform == 'windows' and host_platform != 'windows':
 
 
 # find default_llvm value
-if 'LLVM' in os.environ:
+if 'LLVM' in os.environ or 'LLVM_CONFIG' in os.environ:
     default_llvm = 'yes'
 else:
     default_llvm = 'no'