The _DOSERROR struct in the "dos.h" header file:

struct _DOSERROR {
  int  exterror;
  char class;
  char action;
  char locus;
};
#define DOSERROR _DOSERROR

The second member "char class" uses the C++ keyword "class" and
causes an error when compiling with gpp.

Redefine the _DOSERROR struct using a name other than "class",
e.g. errorclass, errclass, category, etc.

Edit dos.h and rename the offending member.