If neither <filesystem> nor <experimental/filesystem> is detected,
then require <boost/filesystem.hpp>.
If user defines BOOST_GIL_IO_USE_BOOST_FILESYSTEM macro,
then <boost/filesystem.hpp> is pre-selected and required,
and search for any of the C++ standard implementation is skipped.
Remove end-user macro BOOST_GIL_IO_ADD_FS_PATH_SUPPORT
Require tests to always build with support of either
detected C++ filesystem or pre-selected Boost.Filesystem.
Closes ##222
Move test sources to format-specific directories.
Add FIXME and TODO comments about issues discovered while building the format-specific tests.
Add missing `#include`-s with standard library headers.
Apply minor corrections and tidying up.
Continues #459Closes#461