fix(build): define _BSD_SOURCE

System endian conversion macros are gated behind this feature flag for
older versions of GLIBC. `_BSD_SOURCE` and `_SVID_SOURCE` were
deprecated and replaced with `_DEFAULT_SOURCE` starting with GLIBC 2.19.

Package.swift

@@ -27,6 +27,7 @@ let package = Package(
                         .headerSearchPath("src"),
                         .define("_POSIX_C_SOURCE", to: "200112L"),
                         .define("_DEFAULT_SOURCE"),
+                        .define("_BSD_SOURCE"),
                         .define("_DARWIN_C_SOURCE"),
                 ]),
     ],