Update BUILD.gn to remove uclibc hack

We no longer need to build against uclibc and _BSD_SOURCE angers newer gcc toolchains.

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 27cad4b..d9f5160 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -24,10 +24,7 @@
   ]
   if (is_posix) {
     cflags_c = [ "-std=c99" ]
-    defines += [
-      "_BSD_SOURCE",  # Needed for broken uclibc headers.
-      "_XOPEN_SOURCE=700"
-    ]
+    defines += [ "_XOPEN_SOURCE=700" ]
   }
 }