Linux/POSIX 1003.1 API to check /proc/self/auxv avoiding buffer allocation
and other library calls (ported from libpng15).

directory while building with a parallel make.

which demonstrates the tokenization problem previously avoided by using /lib/cpp.
Since all .dfn output is now protected in double quotes unless it is to be macro
substituted the fix should work everywhere.

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

in #ifdef PNG_GET_PALETTE_MAX_SUPPORTED block, and revised pnglibconf.h
and pnglibconf.h.prebuilt accordingly.

internal 'prefix.h' file can only be generated correctly after
pnglibconf.h, however the dependency was not in Makefile.am.  The
symptoms are unpredictable depending on the order make chooses to
build pngprefix.h and pnglibconf.h, often the error goes unnoticed
because there is a system pnglibconf.h to use instead.

png_set_* APIs