autopkgtest regressions due to undefined references to libpng symbols

[Nick Rosbrook]

autopkgtest log: https://autopkgtest.ubuntu.com/results/autopkgtest-kinetic/kinetic/amd64/o/ocrad/20220621_090726_a9748@/log.gz

The autopkgtest for ocrad builds a binary called ocradcheck, but the test script does not pass the correct linker flags. This results in the following:

autopkgtest [09:07:15]: test tests: - - - - - - - - - - stderr - - - - - - - - - -
/usr/bin/ld: /tmp/ccUepqxI.o: in function `main':
/tmp/autopkgtest.EYbpkq/build.7eG/src/ocradcheck.cc:85: undefined reference to `Arg_parser::Arg_parser(int, char const* const*, Arg_parser::Option const*, bool)'
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/11/../../../x86_64-linux-gnu/libocrad.a(png_io.o): in function `read_check_png_sig8(_IO_FILE*, int)':
(.text+0x83): undefined reference to `png_sig_cmp'
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/11/../../../x86_64-linux-gnu/libocrad.a(png_io.o): in function `Page_image::write_png(_IO_FILE*, unsigned int) const':
(.text+0x203): undefined reference to `png_create_write_struct'
/usr/bin/ld: (.text+0x219): undefined reference to `png_create_info_struct'
/usr/bin/ld: (.text+0x23d): undefined reference to `png_set_longjmp_fn'
/usr/bin/ld: (.text+0x263): undefined reference to `png_destroy_write_struct'
/usr/bin/ld: (.text+0x31b): undefined reference to `png_init_io'
/usr/bin/ld: (.text+0x36b): undefined reference to `png_set_IHDR'
/usr/bin/ld: (.text+0x386): undefined reference to `png_set_rows'
/usr/bin/ld: (.text+0x39c): undefined reference to `png_write_png'
/usr/bin/ld: (.text+0x3ab): undefined reference to `png_destroy_write_struct'
/usr/bin/ld: (.text+0x458): undefined reference to `png_destroy_write_struct'

[ ... ]