fix: OSX build steps and README updates.

[dougch]

*Issue #401 *

Description:

Build script for macOS and updates to the README for Mac to match the script.

Tests passing on OSX 10.14, cpp-sdk v1.7.163 and openssl@1.1 installed from brew.

PhaseScriptExecution CMake\ PostBuild\ Rules /var/tmp/build-20191004/build-csdk/aws-encryption-sdk.build/Debug/RUN_TESTS.build/Script-94C362D9F28F4E1F9AC4A310.sh
    cd /var/tmp/build-20191004/aws-encryption-sdk-c
    /bin/sh -c /var/tmp/build-20191004/build-csdk/aws-encryption-sdk.build/Debug/RUN_TESTS.build/Script-94C362D9F28F4E1F9AC4A310.sh
/usr/local/Cellar/cmake/3.15.2/bin/ctest --force-new-ctest-process -C Debug
Test project /private/var/tmp/build-20191004/build-csdk
      Start  1: t_cpputils.test
 1/21 Test  #1: t_cpputils.test ..................   Passed    1.12 sec
      Start  2: t_kms_keyring.test
 2/21 Test  #2: t_kms_keyring.test ...............   Passed    1.10 sec
      Start  3: cipher
 3/21 Test  #3: cipher ...........................   Passed    7.20 sec
      Start  4: header
 4/21 Test  #4: header ...........................   Passed    0.02 sec
      Start  5: materials
 5/21 Test  #5: materials ........................   Passed    0.02 sec
      Start  6: enc_ctx
 6/21 Test  #6: enc_ctx ..........................   Passed    0.04 sec
      Start  7: encrypt
 7/21 Test  #7: encrypt ..........................   Passed    0.93 sec
      Start  8: hkdf
 8/21 Test  #8: hkdf .............................   Passed    0.01 sec
      Start  9: raw_aes_keyring
 9/21 Test  #9: raw_aes_keyring ..................   Passed    0.01 sec
      Start 10: raw_rsa_keyring
10/21 Test #10: raw_rsa_keyring ..................   Passed    0.03 sec
      Start 11: multi_keyring
11/21 Test #11: multi_keyring ....................   Passed    0.01 sec
      Start 12: signature
12/21 Test #12: signature ........................   Passed    3.73 sec
      Start 13: trailing_sig
13/21 Test #13: trailing_sig .....................   Passed    0.02 sec
      Start 14: local_cache
14/21 Test #14: local_cache ......................   Passed    0.04 sec
      Start 15: caching_cmm
15/21 Test #15: caching_cmm ......................   Passed    0.04 sec
      Start 16: keyring_trace
16/21 Test #16: keyring_trace ....................   Passed    0.01 sec
      Start 17: decrypt_aes128_hkdf
17/21 Test #17: decrypt_aes128_hkdf ..............   Passed    1.07 sec
      Start 18: decrypt_aes128_hkdf_bad_header
18/21 Test #18: decrypt_aes128_hkdf_bad_header ...   Passed    0.01 sec
      Start 19: decrypt_hello
19/21 Test #19: decrypt_hello ....................   Passed    0.01 sec
      Start 20: decrypt_hello_tiny
20/21 Test #20: decrypt_hello_tiny ...............   Passed    0.01 sec
      Start 21: decryption_vectors
21/21 Test #21: decryption_vectors ...............   Passed   14.55 sec

100% tests passed, 0 tests failed out of 21

Total Test time (real) =  30.02 sec

** BUILD SUCCEEDED **

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