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>

scons/gallium.py

@@ -410,7 +410,7 @@ def generate(env):
         # Work around aliasing bugs - developers should comment this out
         ccflags += ['-fno-strict-aliasing']
         ccflags += ['-g']
-        if env['build'] in ('checked', 'profile'):
+        if env['build'] in ('checked', 'profile') or env['asan']:
             # See http://code.google.com/p/jrfonseca/wiki/Gprof2Dot#Which_options_should_I_pass_to_gcc_when_compiling_for_profiling?
             ccflags += [
                 '-fno-omit-frame-pointer',
@@ -540,6 +540,16 @@ def generate(env):
             # scan-build will produce more comprehensive output
             env.Append(CCFLAGS = ['--analyze'])
 
+    # https://github.com/google/sanitizers/wiki/AddressSanitizer
+    if env['asan']:
+        if gcc_compat:
+            env.Append(CCFLAGS = [
+                '-fsanitize=address',
+            ])
+            env.Append(LINKFLAGS = [
+                '-fsanitize=address',
+            ])
+
     # Assembler options
     if gcc_compat:
         if env['machine'] == 'x86':