GridLABD: New Package

[pedromxavier]

This is meant to help distributing GridLAB-D to multiple platforms as a GridLABD_jll package that will later be wrapped by a more complete Julia interface around its command-line functionality.

Current Issues (why the manual PR)

• When building to some platforms, the build was successful at every point, but interrupted by an EOFError at the last step, which I couldn't trace to anything specific.

[pedromxavier]

This patch solves the problem that allows compilation to succeed in Darwin platforms, but still this requires APPLE to be TRUE in CMake or CMAKE_SYSTEM_NAME to be Darwin instead of Linux when cross-compiling for apple platforms.
To make it work locally, I had to explicitly set -DCMAKE_SYSTEM_NAME=Darwin as an extra option in the script string for build_tarballs.