Fix libbsd's cdefs.h to be compatible with gcc 4.8.x (#4979)

* Fix cdefs macro to be compatible with gcc 4.8.x

See the discussion in #4945 (after the merge) for additional
background.

libbsd builds with gcc@5.4.0 on CentOS 7, but not with the system's
gcc@4.8.5.  Others have reported problems with gcc@4.8.3 on Fedora 19.

The problem boils down to the lack of support for the clang extension
`__has_include_next`.  The immediate symptom seems to be the
pre-processor using defining macro like this

```
```

then then tripping over an expansion of it like this:

```
blah.h:13:23: error: missing binary operator before token "("
```

This patch changes the macro definition to:

```
```

which swallows the arguments with which the macro is invoked.

The end result is that libbsd builds for me on CentOS 7 using the
system compiler.

* Apply this patch for any compiler version before 5

This includes subversions of 4, like 4.8.5.

0 files changed, 0 insertions, 0 deletions