Update Mac OSX build instructions

 * Added new dependencies (icu, autotools)
 * Indicate that `icu-config` must be on path
 * sigar symlink workaround no longer necessary.

Change-Id: I8176972133bef14e4fc48616656dba9b55921c80
Reviewed-on: http://review.couchbase.org/26295
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>