configure: Fix the _BSD_SOURCE define for the Haiku build

The Haiku VM that we are going to add is using _BSD_SOURCE instead
of BSD_SOURCE (without initial underscore)... according to David
Carlier, the BSD_SOURCE without underscore was likely a typo, so
let's simply add the underscore there now.
This fixes the build failure with the bswapXX() macros not being
defined after including <endian.h>.

Message-Id: <20201114165137.15379-2-thuth@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>