Add --with-homebrew-openssl option to serf

However this does not seem to work.

The built library still uses the default system ones despite that sqlite3 and "-I/usr/local/opt/sqlite/include" is specified when building.

```bash
$ otool -L /usr/local/Cellar/serf/1.3.0/lib/libserf-1.dylib
/usr/local/Cellar/serf/1.3.0/lib/libserf-1.dylib:
	/usr/local/lib/libserf-1.dylib (compatibility version 4.0.0, current version 4.0.0)
	/usr/lib/libssl.0.9.8.dylib (compatibility version 0.9.8, current version 47.1.0)
	/usr/lib/libcrypto.0.9.8.dylib (compatibility version 0.9.8, current version 47.1.0)
	/usr/lib/libz.1.dylib (compatibility version 1.0.0, current version 1.2.5)
	/usr/lib/libapr-1.0.dylib (compatibility version 5.0.0, current version 5.5.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 169.3.0)
	/usr/lib/libaprutil-1.0.dylib (compatibility version 4.0.0, current version 4.12.0)
	/usr/lib/libexpat.1.dylib (compatibility version 7.0.0, current version 7.2.0)
	/usr/lib/libiconv.2.dylib (compatibility version 7.0.0, current version 7.0.0)
	/usr/lib/libsqlite3.dylib (compatibility version 9.0.0, current version 9.6.0)
	/System/Library/Frameworks/LDAP.framework/Versions/A/LDAP (compatibility version 1.0.0, current version 2.4.0)
```

This is potentially a problem for me because the system openssl is too old, that if try to access a https repository with self-signed certificate, svn will simply quit rather than asking whether to accept the certificate.