stage2: support ZIG_LIBC env var and detect self as system C compiler
stage2: support ZIG_LIBC env var and detect self as system C compiler

raulgrell/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,andrewrk/zig