configure: Check for actual functions in libraries with AC_CHECK_LIB

Checking for `main` produces code like this in the test program:

  int
  main ()
  {
  return main ();
    ;
    return 0;
  }

This recursive call results in a warning message with some compilers (e.g.
Clang in newer Xcode versions: "all paths through this function will call
itself [-Winfinite-recursion]"), which lets the tests fail when compiling
with -Werror.

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