[C backend] Support GCC 4.9+ eventually

[Related to #19, which is the same thing for clang.]

This should be pretty easy, as Intel's changes were into trunk [a year ago](https://software.intel.com/en-us/articles/cilk-plus-array-notation-for-c-accepted-into-gcc-mainline) and [released in GCC 4.9](https://gcc.gnu.org/gcc-4.9/changes.html). I'm not sure, but I assume that this pragma simd stuff is unrelated to the [previous `tree-ssa` auto-vectorization effort](http://gcc.gnu.org/projects/tree-ssa/vectorization.html)?

The remaining holdup would seem to be that those release notes mention supporting all of the Cilk 1.2 ABI except for `_Cilk_for`. We need `_Cilk_for`.