FcConfigSubstitute - Execute substitutions
#include <fontconfig/fontconfig.h>
FcBool FcConfigSubstitute (FcConfig *config, FcPattern *p, FcMatchKind kind);
Calls FcConfigSubstituteWithPat setting p_pat to NULL. Returns FcFalse if the
  substitution cannot be performed (due to allocation failure). Otherwise
  returns FcTrue. If config is NULL, the current configuration is used.