Evaluate reference to _externals.d.ts in generated skylark.d.ts

[mistaecko]

TypeScript 0.9.1-1 writes 'references' to .d.ts files to the generated skylark.d.ts. When compiling other sources against this skylark.d.ts the compiler will generate an error/warning because it cannot find these taken-over references. In our case, _externals.d.ts is, however, not required for compiling apps/games that use Skylark.