Fix for rustc update 2014-12-15
Correct output for empty structs.

This fixes a bug where empty C structs would be outputted as

    struct Struct_Foo {
    }

instead of

    struct Struct_Foo;
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.