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).

Too many input files would overflow nfiles
Signed-off-by: NJohn Bowler <jbowler@acm.org>

timepng would could a file as added even if it failed and the assembly file got
rewound.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

Signed-off-by: NJohn Bowler <jbowler@acm.org>

Signed-off-by: NJohn Bowler <jbowler@acm.org>

Signed-off-by: NJohn Bowler <jbowler@acm.org>

Signed-off-by: NJohn Bowler <jbowler@acm.org>

Signed-off-by: NJohn Bowler <jbowler@acm.org>

PNG_FILTER_NONE|PNG_FILTER_SUB|PNG_FILTER_UP).

    - memcpy-free implementations of load3() / store3().
    These should have less variance compiler to compiler.

    - call load3() only when needed at the end of a scanline.
    In the middle, we can use the faster load4(), ignoring that byte.

Without these flags pnglibconf.c compilation fails because it
includes zlib.h -> zconf.h -> sys/types.h which is not found until we
give a correct SDK path with -isysroot flag.

Without this patch, CMAKE_C_FLAGS will be processed as a
string/filename on MSVC (e.g. when CMAKE_C_FLAGS is equal
to " /D_WIN32 /D_WINDOWS /W3") and the whole compilation fails.

For example, ZLIB_INCLUDE_DIR might include the directory where zlib.h
is located (source dir) and the zconf.h file is located (binary dir).