Add support for rdmd in gdc + set $RDMD variable

[wilzbach]

• unifies scripting because now it can be assumed that rdmd exists and that $RDMD will point to it.

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[wilzbach]

Urgh. Some GDC versions have lower Phobos versions despite having a higher release number :/

[wilzbach]

Also in relation with https://github.com/dlang/installer/pull/297 it might be interesting to add support for a --rdmd flag.

[AndrewEdwards]

@wilzbach - Still moving forward? Please resolve conflicts.