lang/zig-devel: NEW PORT - latest version of zig language

[dch]

Summary

• based off https://reviews.freebsd.org/D41067
• with further changes from lang/zig which has changed in between

This adds latest development version of zig-lang - upcoming 0.12.
Makefile was based on lang/zig Makefile and simplified.

lang/zig-devel: align with lang/zig Makefile

Test Plan

Tested on 14.0-RELEASE and 15.0-CURRENT.

[agh_riseup.net]

lang/zig-devel/Makefile

65

s,mkdir,${MKDIR}, please?

[dch]

fix lower-case mkdir per agh@'s comment