Add support for Mac

[bararchy]

Right now the shard was tested, built and ran only on Linux, but we should let the Mac guys in on the fun :)

Suggested way to do it is by having compile time OS specific code wrapped so that the program can be used in different operating systems.

For example (from crystal repo)

{% if flag?(:freebsd) || flag?(:openbsd) %}
  require "./system/unix/sysctl_cpucount"
{% elsif flag?(:unix) %}
  require "./system/unix/sysconf_cpucount"
{% end %}

[j8r]

Using macros is the way to go. I think also it would be good to put each platform specific implementation in its directory like linux, bsd... and of course keeping the same API.

[fridgerator]

+1 for mac support

[Dmdv]

+1 for Mac
Should be more generic rather using /proc/meminfo
Also, working not on every Linux