add srcDir variable to nim.cfg

[Graveflo]

There might be a way to do this but I couldn't find anything about it. This is a very simple thing that goes a long way in certain situations. Trying to avoid needing to switch to nimscript just to get:

# config.nims
import os
let srcDir = currentSourcePath.parentDir()
switch("define", &"ProjPath:\"{srcDir}\"")

with this change just needs:

# nim.cfg
d %= "ProjPath=$srcDir"

[Araq]

I don't mind the feature but this seems offplace. Shouldn't it be done somewhere else in the compiler?

[Graveflo]

I looked into it again and it seems the path is determined in loadConfigs which calls readConfigFile (current placement of the change) and then it's promptly handed off to the machinery of while tok.tokType != tkEof: parseAssignment ... where the config file is processed. I don't see where else this could go.