Crystal: Enable `File.join` in Macro

[xqyww123]

Macro may require to safely join the paths to generate shell commands or others, when the path may be determined by variables. Like this:

macro mark_down(what)
{{ run("cat res/#{what}.md").stringify }} # not cross-platform safe!
# cross-platform safe:
{{ run("cat " + join_path("res", "#{what}.md")).stringify }}
end

puts mark_down book

[asterite]

Just do:

{% if flag?(:windows) %}
...
{% else %}
{% end %}

because I think you will have problems with cat working on Windows to start with...

[Sija]

Once there was #2791, which would solve this issue.

[xqyww123]

Okay, but the case of executing a program locates in a specific path? In this case, path join may be required.

[RX14]

@asterite that doesn't work when cross-compiling, since the macros are executed on the host triple and the flags are for the target triple.

[asterite]

Well, nobody liked my idea of the read_file macro method. If you want it now, go for it :-)