-D_DEFAULT_SOURCE *and* -D_GNU_SOURCE together resolve all warnings
about implicitly defined functions.
-D_LARGEFILE_SOURCE and -D_FILE_OFFSET_BITS=64 enable gcc's Large File
Extensions to allow a 32-bit host to serve files larger than 2GB.
Based on what Makefile.boot does, for platforms without bmake:
http://cvsweb.netbsd.org/bsdweb.cgi/src/libexec/httpd/Makefile.boot