pnglibconf.h generated with these lines:
PATH=$DEVKITPPC/bin:$PATH
CPPFLAGS=-I$DEVKITPRO/portlibs/ppc/include LDFLAGS=-L$DEVKITPRO/portlibs/ppc/lib ./configure --prefix=$DEVKITPRO/portlibs/ppc --host=powerpc-eabi --disable-shared --enable-static
make