Currently only the poppler glib interface is used.

This is not quite perfect, as it does not allow setting the error
"handler", and the default one outputs to stderr.

Debian provides libpoppler-private-dev including poppler/Error.h with

  void setErrorFunction(void (* f)(int , char *, va_list args));

for Poppler versions before 0.20 and

  void setErrorCallback(void (*cbk)(void *data, ErrorCategory category,
                                    int pos, char *msg),
                        void *data);

for Poppler 0.20 or later.

Instead of relying on the availability of the private header, we test
whether we can use setErrorFunction() or setErrorCallback() when adding
the above declarations and -lpoppler when linking.  If one of these is
successful, we register our own error function/callback which instead of
writing to stderr gathers error messages in a character vector which can
returned as the "errors" attribute of the object computed, and used by 
print() methods as needed.

[This is a bit of a nuisance, as poppler-glib is C but the setting the 
error function/callback must be done in C++ ...]