I am trying to graph both FresnelC(x) and FresnelS(x). There is this example which says that libcerf must be used but I am unsure how to use it. When I run gnuplot and try
plot [-4:4] FresnelC(x)
I get the message
undefined function: FresnelC
How do I enable libcerf with gnuplot?
If your gnuplot executable reports +LIBCERF as part of the output from show version long then it was indeed configured with the cerf library.
However the FresnelC function is part of the greatly expanded support for complex special functions provided in the development version (gnuplot 5.5). It is not present in any of the "stable" releases through 5.4. Note that the demo page you linked to is from the demo collection for the development version.
A summary of new features introduced in the development version is here: Development Version