If searchIdx() doesn't find the id it returns -1, which breaks
motioncelssvector.at (idx). Check for it and return if not found.
Changing a few other lines for style consistency.
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
