Added the detail/shared.hpp header (currently contains only the BOOST_GIL_THROW_THROUGH_C_SUPPORTED macro definition).
Added BOOST_GIL_THROW_THROUGH_C_SUPPORTED support to the LibPNG backend.
Removed some comented out code.
Added custom error, warning, read and write functions/callbacks that together with the added support for PNG_NO_STDIO, PNG_NO_ERROR_TEXT and PNG_NO_WARNINGS enabled the removal of stdio-printf family of functions from the binary.