tune g++ to reduce mem usage

modify bde default.opts from https://github.com/bloomberg/bde/pull/99

Note: not sure how to differentiate clang from gcc in default.opts.  If '--param ggc-min-expand=30' flag is passed to clang, it will result in a warning and will be ignored:
clang-3.4: warning: argument unused during compilation: '--param gcc-min-expand=30'