What is a good function to use in the AC_CHECK_LIB for the fltk library? I have a project that uses fltk, and I would like to setup a configure.ac to check that the library exists. AC_CHECK_LIB takes the library name, plus a function name to check. I would like a C function is possible to simplify things, and a function that exists in all / most releases/version of fltk.
Well, you should not use AC_CHECK_LIB in the first place.
Use AC_CHECK_LIBS.
Documentation on how to do this is here: https://autotools.io/autoconf/finding.html