scons: Allow building with Address Sanitizer.

libasan is never linked to shared objects (which doesn't go well with
-z,defs).  It must either be linked to the main executable, or (more
practically for OpenGL drivers) be pre-loaded via LD_PRELOAD.

Otherwise works.

I didn't find anything with llvmpipe.  I suspect the fact that the
JIT compiled code isn't instrumented means there are lots of errors it
can't catch.

But for non-JIT drivers, the Address/Leak Sanitizers seem like a faster
alternative to Valgrind.

Usage (Ubuntu 15.10):

   scons asan=1 libgl-xlib
   export LD_LIBRARY_PATH=$PWD/build/linux-x86_64-debug/gallium/targets/libgl-xlib
   LD_PRELOAD=libasan.so.2 any-opengl-application

Acked-by: Roland Scheidegger <sroland@vmware.com>
diff --git a/common.py b/common.py
index 7a93941..70e6708 100644
--- a/common.py
+++ b/common.py
@@ -97,6 +97,7 @@
     opts.Add(BoolOption('embedded', 'embedded build', 'no'))
     opts.Add(BoolOption('analyze',
                         'enable static code analysis where available', 'no'))
+    opts.Add(BoolOption('asan', 'enable Address Sanitizer', 'no'))
     opts.Add('toolchain', 'compiler toolchain', default_toolchain)
     opts.Add(BoolOption('gles', 'EXPERIMENTAL: enable OpenGL ES support',
                         'no'))