Use _DEFAULT_SOURCE instead of _BSD_SOURCE

Due to the ``Makefile``'s ``-Werror`` invocation, I get the following error when compiling:

    #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [-Werror=cpp]

This commit changes ``-D_BSD_SOURCE`` to ``-D_DEFAULT_SOURCE`` which solves the issue quite nicely.