getoptlong() is part of the GNU C Library. Try
http://www.gnu.org/software/libc/ or
http://ftp.gnu.org/gnu/glibc/ .
You might also find the GNU
Portable C library helpfull - try
http://www.gnu.org/software/gnulib/
Off topic, Off topic, Off topic!
See, you're not even allowed to say that:
getoptlong() is part of the GNU C Library.
Because, you see, it may or may not be. No one here is allowed to know
whether it is or is not. Further, even if it is, it might also be part
of the "shenornigan C library" or the "zooziwhat C library" as well as
57 other C libraries, so saying it is part of the "GNU C Library"
(whatever that may be...) is just, so, like, totally Off Topic.
And you don't want that.