AC_CHECK_HEADERS() calls action-if-not-found also if just one of
the headers checked for is missing, which is not what we wanted.
Also, check for highgui_c.h instead of highgui.hpp.
https://bugzilla.gnome.org/show_bug.cgi?id=672226
Where highgui.h doesn't exist any more, but only opencv2/highgui/highgui.hpp.
Also, not quite sure why we're checking for cvaux.h, it's not used anywhere.