Build fails with "--with-system-poppler"

[Caolán McNamara]

Prior to:
commit 697804f48f4217dab145d96175c6cddf560ec614
Date:   Thu Apr 16 10:13:06 2015 +0200

    Make cpp/poppler-version.h header optional

we errored out if --with-system-poppler was used and there was no
cpp/poppler-version.h

That was 6 years ago, so it's possible that restoring that error on not
seeing cpp/poppler-version.h and changing the min version of poppler to
whichever one introduced cpp/poppler-version.h would work nowadays
without controversy and avoid the loophole you tripped up in.