options. This makes the installed pnglibconf.h more readable but causes no
other change. The intention is that users of libpng will find it
easier to understand if an API they need is missing.
This allows the functions
to read and write ints to be disabled independently of PNG_USE_READ_MACROS,
which allows libpng to be built with the functions even though the default
is to use the macros - this allows applications to choose at app build
time whether or not to use macros (previously impossible because the
functions weren't in the default build.)
