rustbuild: can't remove dependency on FileCheck

[japaric]

With the old build system, one can use the --disable-codegen-tests flag to disable the check for the existence of the FileCheck binary.. Passing this flag to rustbuild doesn't have the same effect; it still checks for the FileCheck binary.

cc @alexcrichton

[alexcrichton]

I believe the configure script has some variable for this, so should be easy enough to detect!