Correctly detect infinite recursion in function application

If we're evaluating some application ‘v = f x’, we can't store ‘f’
temporarily in ‘v’, because if ‘f x’ refers to ‘v’, it will get ‘f’
rather than an infinite recursion error.

Unfortunately, this breaks the tail call optimisation introduced in
c897bac549.

Fixes #217.

src/libexpr/eval.cc

@@ -779,8 +779,10 @@ void ExprLambda::eval(EvalState & state, Env & env, Value & v)
 
 void ExprApp::eval(EvalState & state, Env & env, Value & v)
 {
-    e1->eval(state, env, v);
-    state.callFunction(v, *(e2->maybeThunk(state, env)), v);
+    /* FIXME: vFun prevents GCC from doing tail call optimisation. */
+    Value vFun;
+    e1->eval(state, env, vFun);
+    state.callFunction(vFun, *(e2->maybeThunk(state, env)), v);
 }