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).
Silence 'unused parameter' build warnings (Cosmin).
$(CP) is now used alongside $(RM_F). Also, use 'copy' instead
of 'cp' where applicable, and applied other minor makefile changes.
Don't warn about invalid dimensions exceeding user limits.
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.
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.
