Also apply minor updates to makefile.gcc and makefile.msys

makefiles. This does not work on some versions of make; bug introduced in
libpng version 1.6.11.

header with a custom header, via the make PNGLIBCONF_H_PREBUILT
macro.

of 'cp' where applicable, and applied other minor makefile changes.

problems with build scripts that search for config.h and find an
inappropriate one in the standard paths.

from the source files, along with "#define PNG_EXPOSE_INTERNAL_STRUCTURES"
and "#define PNG_NO_PEDANTIC_WARNINGS" (John Bowler).
Also created new pngdebug.h and moved debug definitions there.

and added "#define PNG_NO_PEDANTIC_WARNINGS" in the libpng source files.

about deprecated function use and deprecated structure access unless the
user defines PNG_PEDANTIC_WARNINGS.  Added "#define PNG_NO_PEDANTIC_WARNINGS"
in the libpng source files.  Removed "-DPNG_CONFIGURE_LIBPNG" from the
makefiles and projects since the warnings no longer depend on this.

ported from libpng-1.4.0

to prevent warnings about direct access to png structs by libpng functions
while building libpng.  They need to be tested, especially
those using compilers other than gcc.

These are mostly just cosmetic changes.

It is no longer used since the MMX support was removed.