clock() provides approximation of processor time used, measured in "clocks".
Amount of clock per second varies between 100 (Minix, NetBSD, OpenBSD), 128
(DragonFly BSD, FreeBSD, older versions of MacOS X), 1000 (Windows) and 1000000
(Linux, newer versions of MacOS X, Solaris). The range of these numbers make
conversion to milliseconds (the unit of display in `qrtest`) problematic.
clock_gettime() with its nanosecond time base appears to be a better and more
portable alternative to clock().
Before this patch check_if_png() comment was kind of misleading because
it talked about the behaviour of png_sig_cmp() which isn't the same as
check_if_png().
