FcConfigFilename - Find a config file
#include <fontconfig/fontconfig.h>
FcChar8 * FcConfigFilename (const FcChar8 *name);
Given the specified external entity name, return the associated filename. This
  provides applications a way to convert various configuration file references
  into filename form.
A null or empty name indicates that the default
    configuration file should be used; which file this references can be
    overridden with the FONTCONFIG_FILE environment variable. Next, if the name
    starts with ~, it refers to a file in the current users home
    directory. Otherwise if the name doesn't start with '/', it refers to a file
    in the default configuration directory; the built-in default directory can
    be overridden with the FONTCONFIG_PATH environment variable.