tune g++ to reduce mem usage

g++ takes over 3 GB to compile bslstl_hashtable.t.cpp
'g++ --param ggc-min-expand=50' lowers this to about 2 GB by running
the g++ internal garbage collection more frequently than the default.
On a dual-core laptop with 4 GB of memory and no swap space, building
with make -j 3 was successful, though building with make -j 4 thrashed
due to increased memory demands of extra job.  make -j 3 combined with
g++ --param gcc-min-expand=30 was both faster and took less CPU than
building with make -j 4 combined with g++ --param ggc-min-expand=30.