Index: test/plugins/builder_base.py
===================================================================
--- test/plugins/builder_base.py
+++ test/plugins/builder_base.py
@@ -78,8 +78,29 @@
     # If d is None or an empty mapping, just return an empty string.
     if not d:
         return ""
-    pattern = '%s="%s"' if "win32" in sys.platform else "%s='%s'"
-    cmdline = " ".join([pattern % (k, v) for k, v in d.items()])
+
+    def escape(s, escape_chars):
+        """
+        Returns a string with the specified characters escaped by a backslash.
+        """
+        s = s.replace('\\', '\\\\')
+        for c in escape_chars:
+            s = s.replace(c, '\\' + c)
+        return s
+
+    def setOrAppendVariable(k, v):
+        """
+        Append to EXTRAS variables as clobbering them unsets extra flags passed
+        for cross compilation.
+        """
+        quote = '"' if "win32" in sys.platform else "'"
+        append_vars = ["CFLAGS_EXTRAS", "LD_EXTRAS"]
+        if k in append_vars and os.environ.has_key(k):
+            v = os.environ[k] + " " + v
+
+        return '%s=%s%s%s' % (k, quote, escape(v, quote), quote)
+
+    cmdline = " ".join([setOrAppendVariable(k, v) for k, v in d.items()])
 
     return cmdline