Adding parsing and sema tests for _Cilk_for statements

- tests with templates are not included.
Fix is*Decl for the case where the Decl is not set

- Was causing spurious assertion failures
Install libcilkrts and cilk headers with autoconf

- Places libcilkrts.* into the build and install directories, just as is
  already done in cmake
- Installs include/cilk into the build and install directories, as is
  already done with cmake