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.

interface I {}
class A : I {}
class B : A {}

void main () {
    A a = new A;
    I i = a;

    assert(!cast(B)a);
    assert(!cast(B)i);
}