Factor out the code to find gcc, and add a command-line option to override the default executable name. This should help cross-compilation by allowing a cross-gcc to be specified from the configuration file instead of only through the CC environment variable.

module nested12;

extern(C) int printf(char*, ...);

void main()
{
    func();
}

void func()
{
    void a(int i)
    {
        printf("%d\n", i);
    }

    void b()
    {
        a(42);
    }

    b();
}