build: add a check_program check

This wraps waf's find_program in our own check boilerplate code so that it
can be used in the declarative dependencies section of the wscript.

Can be used like this:

    }, {
        'name': 'sed',
        'desc': 'sed program',
        'func': check_program('sed', 'SED'),
    }, {

First argument is the program name, and the second is the waf variable name
where the program path will be stored. In this example we will be able to
refer to sed with ${{SED}} when creating waf Tasks in wscript_build.

/cc @giselher: I think you need this for wayland-scanner.