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.
Adds utilities that allow text comparison of bindings.

This is unused in this branch.  Just preparation for upcoming branches
that will feature tests.
Improves argument name parsing in function pointers.

Previously, all function pointer types always had their arguments bound
as arg1, arg2, etc.  This change preserves the argument names, when they
are available.

parser.rs
- Uses the type cursor to visit children and gather argument names and
  types.

tests/func_ptr.rs
- Binds tests/headers/func_ptr.h and tests/headers/func_ptr_in_struct.h
  and ensures that argument names are correctly bound.