Overhaul mysql.buildlink3.mk

[Jonathan Perkin <jperkin%joyent.com@localhost>]

With the renewed interest in our MySQL infrastructure, it's about time I 
merged some patches I've been sitting on for far too many years.  As 
some of you know, I added various MySQL forks, primarily Percona, and 
support for multi packages.

Here's an initial cut at an updated mysql.bl3.mk:

  https://gist.github.com/jperkin/868f928d795b6b775e52013416e63623

It's not quite the same as what we have in our fork, I took the time to 
update it a little, and as yet it doesn't support all the extra Percona 
packages or multi packages.  It does however:

  - Avoid shouting in package names (the shouty names are still 
    supported for those using them).

  - Switch to proper pkg_info checks for if a package is installed, 
    rather than hardcoding a bunch of library names to look for.

  - Abstract away the differences between each version, making adding 
    new ones trivial.

  - Simplify the version selection.

Once this is in I can look at merging in some of the newer Percona 
packages.  We'll leave the multi package support to later, as that's a 
bit more invasive.

Cheers,

--
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com