Mark Bixby writes in response to a question from Kevin Newman:

>The next release of BIND/iX that I'm still debugging defines its own
>strdup()

.. and

>extern char *strdup(const char *);

strdup is one of those missing routines that "everyone" needs. GNU
contains a library called libiberty.a (I've included it in more
recent releases of GNU in /usr/local/lib). This includes such
goodies as:

argv.o         floatformat.o  index.o        spaces.o       xexit.o
asprintf.o     fnmatch.o      objalloc.o     strdup.o       xmalloc.o
basename.o     getopt.o       obstack.o      strerror.o     xstrdup.o
choose-temp.o  getopt1.o      pexecute.o     strsignal.o    xstrerror.o
concat.o       getpagesize.o  random.o       vasprintf.o
cplus-dem.o    getruntime.o   rindex.o       vfork.o
fdmatch.o      hex.o          sigsetmask.o   xatexit.o

Include -liberty and you should have access to strdup() and friends.