old clang compilers (MacPorts Ticket #43939).

0644 to 0755 (Cosmin).

    declarations (Max Stepin).

behavior of the sequential reader and consistent with the manual. The
row_callback now receives the proper pass number and unexpanded rows,
when png_combine_row is not used or not built, by moving some code
out of the PNG_READ_INTERLACING_SUPPORTED blocks.

interferes with "blocky" expansion of sub-8-bit interlaced PNG files.

header with a custom header, via the make PNGLIBCONF_H_PREBUILT
macro.

of 'cp' where applicable, and applied other minor makefile changes.