Merge #838: Make autotools check for all the used openssl functions

3734b68200ee37f5eea80f47d611e9b5a65548fe Configure echo if openssl tests are enabled (Elichai Turkel)
e6692778d3f6507eb1325785cdd424073a945ff7 Modify bitcoin_secp.m4's openssl check to call all the functions that we use in the tests/benchmarks. That way linking will fail if those symbols are missing (Elichai Turkel)

Pull request description:

  I added all the openssl functions that we call in `tests.c` and in `bench_verify.c` to the m4 check, that way if any of them are missing it won't enable openssl.
  I also modified it a little to prevent a segmentation fault when running that program (not that it really matters for autotools)

  This should fix #836

ACKs for top commit:
  sipa:
    ACK 3734b68200ee37f5eea80f47d611e9b5a65548fe
  real-or-random:
    ACK 3734b68200ee37f5eea80f47d611e9b5a65548fe

Tree-SHA512: c82aa96a4176061284dfa5fdb87ca874a25aa2e11f75c4ec6d1edebcc8a19e2bc940990f8a5cfa64776fd295b6fd3a140fa2afede29326564504bc8d1a3a6b69