This is a problem that seems to have appeared with the latest version of the Arduino IDE. The fix is to locate the comment:
// Forward references
and insert the following two lines after it:
void error (PGM_P string);
void pfstring (PGM_P s, pfun_t pfun);
This fix will be incorporated in the next release.
Explanation: normally the Arduino IDE automatically resolves forward references to functions that are defined later in the file, but for some reason it has stopped working for these two references.