[libpng15] The macros passed in the command line to Borland make were ignored if

similarly-named macros are already defined in makefiles. This behavior
is different from POSIX make and other make programs.  Surround the
macro definitions with ifndef guards (Cosmin).
[libpng15] Use memcpy instead of CopyMemory in the definition of png_memcpy.

The Borland C++ build breaks otherwise (Cosmin).
[libpng15] Imported from libpng-1.5.20rc02.tar