error: memunit_step was not declared in this scope

(follow-up to https://github.com/boostorg/gil/pull/173#discussion_r239542235)

### Actual behavior

Compile tests and examples with with `-DBOOST_GIL_USE_CONCEPT_CHECK=1`. For example, add the define to the GIL's top-level `Jamfile`:

```
project boost-gil
:
requirements
BOOST_GIL_USE_CONCEPT_CHECK
...
```

Run `b2 -d 2` to confirm the define is included in the compiler command line:

Observer numerous compiler errors:

```
boost/gil/concepts/pixel_iterator.hpp:87:41: error: ‘memunit_step’ was not declared in this scope
boost/gil/concepts/pixel_iterator.hpp:90:30: error: ‘memunit_advanced’ was not declared in this scope
...
```

None of the `memunit_*` functions have ever been (forward) declared in any way, for example:

https://github.com/boostorg/gil/blob/boost-1.35.0/include/boost/gil/gil_concept.hpp
https://github.com/boostorg/gil/blob/boost-1.60.0/include/boost/gil/gil_concept.hpp
https://github.com/boostorg/gil/blob/boost-1.65.0/include/boost/gil/gil_concept.hpp

### Expected behavior

No compilation error.

### Environment

All relevant information like:

- Boost version (see ``): from 1.35 until 1.69
- Compiler version: GCC 5.5.0 but any C++11 compiler should fail