Force gcc-4.2 compiler when installing Ruby

Avoid errors like:

    st.c:520:35: error: implicit conversion loses integer precision:
    'st_index_t' (aka 'unsigned long') to 'int' [-Werror,-Wshorten-64-to-32]
    i = table->num_entries++; ~ ~~~~~~~~~~~~~~~~~~^~

https://github.com/sstephenson/ruby-build/issues/287