Certain optional gcc warning flags resulted in warnings in libpng code.
With these changes only -Wconversion and -Wcast-qual cannot be turned on.
Changes are trivial rearrangements of code.  -Wconversion is not possible
for pngrutil.c (because of the widespread use of += et al on variables
smaller than (int) or (unsigned int)) and -Wcast-qual is not possible
with pngwio.c and pngwutil.c because the 'write' callback and zlib
compression both fail to declare their input buffers with 'const'.

Moved reading of file signature into png_read_sig (Cosmin)
Fixed atomicity of chunk header serialization (Cosmin)
Added test for io_state in pngtest.c (Cosmin)
Added "#!/bin/sh" at the top of contrib/pngminim/*/gather.sh scripts.

added some whitespace in pngrio.c and pngrutil.c

Also removed two obsolete comments from png.c

to catch library and header mismatch, also found at the end of pngtest.c.

as it was prior to libpng-1.2.16beta2, to simplify version differences.

so it will work with automake versions earlier than 1.11.1

(omitted autogen.sh, ../configure should be ./configure)

to remove obsolete statement about a custom ltmain.sh

in printf statements in pngvalid.c, and changed %ud to %lu.