[libpng15] Enabled renaming of the extern (but not DLL exported) ARM filter

functions.  Also enabled inclusion of pngpriv.h in non-C files by preventing
C/C++ specific definitions if PNG_VERSION_INFO_ONLY is defined.
[libpng15] Added documentation of new png_get_palette_max() function.
[libpng15] Imported from libpng-1.5.15beta01.tar
[libpng15] Bump version to 1.5.15beta02
[libpng15] Made a probably general fix for the preprocessing used to generate

source files. This change might make the check for a special C preprocessor
unnecessary, and it also eliminates the requirement for sed in the
configure build process. NOTE: this is incomplete, a change must also
be made to pnglibconf.mak
[libpng15] Fix typo in Makefile.am, fix scripts/pnglibconf.mak and makefile.std