* 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.
