Thirdparty squeasel build fails on macOS 10.12

Since recent changes to squeasel landed in 8993716d7, the thirdparty build of squeasel has been broken on macOS 10.12. The OpenSSL system headers in /usr/include/openssl are not present in 10.12, and squeasel now depends on them. The error:

<..>/kudu/thirdparty/src/squeasel-c304d3f3481b07bf153979155f02e0aab24d01de/squeasel.c:153:10: fatal error: 'openssl/ssl.h' file not found

An relatively easy workaround is to install OpenSSL via homebrew and set the environment variable EXTRA_CFLAGS="-I/usr/local/opt/openssl/include" when building thirdparty.

We should come up with a long-term solution. tlipcon suggested that we might try to use pkg-config.