SCons: allow setting VALGRINDFLAGS from the environment

tests/SConscript

@@ -92,9 +92,14 @@ artifacts['testsuite'] = ts = ( [ testExecutable(env, dir) for dir in ['lib','co
 #    is explicitly propagated to test.sh
 #
 testEnv = env.Clone()
-if not env['VALGRIND']:
-    testEnv.Append(ENV = { 'VALGRINDFLAGS' : 'DISABLE'
-                         , 'LUMIERA_CONFIG_PATH' : './' })
+
+valgrind = os.environ.get('VALGRINDFLAGS', '') # unset if not defined
+if not valgrind and not env['VALGRIND']:
+    valgrind = 'DISABLE'
+
+testEnv.Append(ENV = { 'VALGRINDFLAGS' : valgrind
+                     , 'LUMIERA_CONFIG_PATH' : './' 
+                     })
     
 testsuites = env['TESTSUITES'] or os.environ.get('TESTSUITES')
 if testsuites: