Allow a blank value in a C constant comment

Right now you can add a C constant comment that declares the constant's value and description. Here's the example from the specs:

```c
/* 0xdeadbeef: This constant is frequently used to indicate a
 * software crash or deadlock in embedded systems. */
rb_define_const(rb_cMask, "DEADBEEF", INT2FIX(MSK_DEADBEEF));
```

There are cases when it is not useful to list the actual value of a constant. For example, in a C extension where a Ruby constant refers to a constant of the same name in the C code. Here's an example from my [gmp gem](https://github.com/srawlins/gmp/blob/master/ext/mprnd.c#L92):

```c
/* MPFR_RNDN: MPFR rounding mode roundTiesToEven */
rb_define_const (mGMP, "MPFR_RNDN", rb_funcall (cGMP_Rnd, new_id, 1, INT2FIX (0)));
```

The `GMP::MPFR_RNDN` constant defined here just holds the value of the [MPFR constant](http://www.mpfr.org/mpfr-current/mpfr.html#Rounding-Modes) `MPFR_RNDN`. In yard, the documentation reveals that the value of `GMP::MPFR_RNDN` is in fact... `MPFR_RNDN`.

This pull request allows the value part of the comment to be blank, in which case the value will not be displayed in the documentation. It allows the following:

```c
/* : MPFR rounding mode roundTiesToEven */
rb_define_const (mGMP, "MPFR_RNDN", rb_funcall (cGMP_Rnd, new_id, 1, INT2FIX (0)));
```