Updates tests in macro.rs and cmath.rs.

macro.rs
- Removes `make_string_vec` in favor of the `vec!` macro.
- Updates `test_parse_process_args` with `vec!`.

cmath.rs
- Adds needed `extern crate libc;` declaration.
- `bindgen!` invocation now references math.h as "/usr/include/math.h",
  which should be pretty universal.  Clang was not able to find the file
  using just "math.h".
- Moves contents of main method into a `#[test]` method.