Initialize ccKey from default cc so defined(gcc) works without --cc.

[gmpreussner]

Fixes #2148

[gmpreussner]

The {.passC: "-Wno-builtin-declaration-mismatch".} in posix.nim sits inside when defined(gcc):. That only takes effect if nimsem sees defined(gcc) as true during semantic analysis.

Nimony’s defined(...) checks config.isDefined(), which treats the C compiler as defined via config.ccKey

The default compiler is "gcc", but ccKey was only set when --cc: was passed explicitly