These inclusions were in place for backward compatibility purposes,
when the headers were split so that code using them would not break.
Make it possible for applications to disable them by defining
LIBBSD_CLEAN_INCLUDES so that buildability can be tested and fixed
before they get removed in a subsequent release.
This means that software being ported should not need to be modified in
the usual case, as the libbsd headers will take over the standard
namespace and fill the missing gaps, and include the system headers.
To use this the new libbsd-transparent.pc file can be used through
pkg-config, which should end up doing the right thing.
