Add support for transparent compilation

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.

include/bsd/sys/cdefs.h

@@ -27,7 +27,11 @@
 #ifndef LIBBSD_CDEFS_H
 #define LIBBSD_CDEFS_H
 
+#ifdef LIBBSD_TRANSPARENT
+#include_next <sys/cdefs.h>
+#else
 #include <sys/cdefs.h>
+#endif
 
 #ifndef __dead2
 # define __dead2