internal functions called by png_do_{read|write}_transformations static. On an
x86-64 DLL build (Gentoo Linux) this reduces the size of the text
segment of the DLL by 1208 bytes, about 0.6%. It also simplifies
maintenance by removing the declarations from pngpriv.h and allowing
easier changes to the internal interfaces.

function definitions before the place where they are called so that
they can be masde static. Move the intrapixel functions and the
grayscale palette builder out of the png?tran.c files. The latter
isn't a transform function and is no longer used internally, and the
former MNG specific functions are better placed in pngread/pngwrite.c

configurations don't build because of missing contrib/libtests test programs
and overly complex dependencies in scripts/pnglibconf.dfa. This change adds
contrib/conftest/*.dfa files that can be used in automatic build
scripts to ensure that these configurations continue to build.

to PNG_SET_UNKNOWN_SUPPORTED in pngpread.c; it led to the possibility of
calling png_handle_unknown() twice.

the #ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED/#endif block.
Removed recently-added #ifdef from pngpread.c; moving the prototype
in pngpriv.h fixed the compiler warning that the #ifdef was trying
to solve.

or intrinsics to be turned on during the build by defining
PNG_ARM_NEON_IMPLEMENTATION to the correct value (2 or 1).  The
default is "undefined" and the build type is selected in pngpriv.h.

pngpriv.h and detect the broken GCC compilers.

off. Also fixed the pngminim makefiles to work when $(MAKEFLAGS) contains stuff
which terminates the make options (as by default in recent versions of Gentoo).

png_chunk_unknown_handling() function, and restored the unknown-chunk
handling code from libpng-1.6.6.

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breaks 'NOREAD' builds.

strip option. pngfix relied on undefined behavior and even a simple change from
gcc to g++ caused it to fail.  The new strip option 'unsafe' has been
implemented and is the default if --max is given.  Option names have
been clarified, with --strip=transform now stripping the bKGD chunk,
which was stripped previously with --strip=unused.
Added all documented chunk types to pngpriv.h

Previous iOS/Xcode fixes for the ARM NEON optimizations moved the test
on __ARM_NEON__ from configure time to compile time.  This breaks symbol
prefixing because the definition of the special png_init_filter_functions
call was hidden at configure time if the relevant compiler arguments are
passed in CFLAGS as opposed to CC.  This change attempts to avoid all
the confusion that would result by declaring the init function even when
it is not used, so that it will always get prefixed.

pnglibconf.h, allowing more of the decisions to be made internally
(pngpriv.h) during the compile.  Without this, symbol prefixing is broken
under certain circumstances on ARM platforms.  Now only the API parts of
the optimizations ('check' vs 'api') are exposed in the public header files
except that the new setting PNG_ARM_NEON_OPT documents how libpng makes the
decision about whether or not to use the optimizations.

Folded some long lines.

to be included for preprocessor definitions only, so it can be used in non-C/C++
files. Back ported from libpng 1.7.