memcpy() with a null source, even if count is zero (Leon Scroggins III).

Coverity rejects code where an array element count has type size_t, this
elminates the code in question from contrib/libtests/pngvalid.c
Signed-off-by: NJohn Bowler <jbowler@acm.org>

The previous version of the code invariably passed just one byte at a time to
libpng.  The intention was to pass a random number of bytes in the range 0..511
(and this is what happens now).
Signed-off-by: NJohn Bowler <jbowler@acm.org>

positions.  Fixed the sequential reader to handle these more robustly
(John Bowler).

properly (patch by Timothy Nikkel).

presence of clock_gettime() in configure.ac and Makefile.am.

on platforms that don't supply clock_gettime().

    Added support to use multiple directories in ZLIBINCDIR variable,
    Fixed CMAKE_C_FLAGS with multiple values when genout is compiled on MSVC,
    Fixed pnglibconf.c compilation on OS X including the sysroot path.

Merge branch 'fix-genout-with-multiple-zlib-inc-dirs' of git://github.com/aseprite/libpng into libpng16

(Bug report by Y.Ohashik).