added gcc.cxxflags to docs

doc/library/config.txt

@@ -150,4 +150,11 @@ Config Attributes
 
     A directory with bin/, lib/, include/ folders containing cuda utilities.
 
+.. attribute:: gcc.cxxflags
+
+    Default: ""
+
+    Extra parameters to pass to gcc when compiling.  Extra include paths,
+    library paths, configuration options, etc.
+