This has something to do with slightly stricter type checking in a new version of GCC or somesuch. I forget where I heard about that.

comment out these two lines in ruby-1.8.6-p111/ext/openssl/openssl_missing.h

120: // int BN_rand_range(BIGNUM *r, BIGNUM *range);
121: // int BN_pseudo_rand_range(BIGNUM *r, BIGNUM *range);

And then try rvm install ruby-1.8.6-p111 again