fix: Automatic detection of <filesystem> header

[marco-langer]

Description

PR #636 has added detection for the <filesystem> header, but now examples using the IO extensions can't be compiled with C++11 or C++14 anymore. This PR fixes this issue.

References

Compiling one of the examples, e.g.

g++ x_gradient.cpp -std=c++11 -ljpeg

results in

In file included from ../../boost/libs/gil/include/boost/gil/io/path_spec.hpp:11,
                 from ../../boost/libs/gil/include/boost/gil/io/get_read_device.hpp:13,
                 from ../../boost/libs/gil/include/boost/gil/io/get_reader.hpp:11,
                 from ../../boost/libs/gil/include/boost/gil/extension/io/jpeg/read.hpp:18,
                 from ../../boost/libs/gil/include/boost/gil/extension/io/jpeg.hpp:11,
                 from ../../boost/libs/gil/example/x_gradient.cpp:9:
../../boost/libs/gil/include/boost/gil/io/detail/filesystem.hpp:55:29: error: ‘filesystem’ is not a namespace-name
   55 | namespace filesystem = std::filesystem;
....

Tasklist

• Add test case(s)
• Ensure all CI builds pass
• Review and approve

[mloskot]

Nice, thanks!

[marco-langer]

I don't understand why it wasn't caught by the tests, are we using BOOST_GIL_IO_USE_BOOST_FILESYSTEM in the test runners?

[mloskot]

@marco-langer

Good question. I don't understand why tests did not catch it. I don't see BOOST_GIL_IO_USE_BOOST_FILESYSTEM defined anywhere.

The GHA basically run b2 test:

gil/.github/workflows/ci.yml

Line 114 in 9ecdb87

./b2 -j3 libs/$LIBRARY/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} variant=debug,release

which only runs minimal IO test called simple_all_formats:

gil/test/extension/io/Jamfile

Lines 36 to 44 in 9ecdb87

	run simple_all_formats.cpp
	: # args
	: # input files
	: # requirements
	<library>/boost/filesystem//boost_filesystem
	[ ac.check-library /libjpeg//libjpeg : <library>/libjpeg//libjpeg : <build>no ]
	[ ac.check-library /zlib//zlib : <library>/zlib//zlib : <build>no ]
	[ ac.check-library /libpng//libpng : <library>/libpng//libpng : <build>no ]
	[ ac.check-library /libtiff//libtiff : <library>/libtiff//libtiff : <build>no ]

So, for C++11 and C++14 none of these #define-s should happen

gil/include/boost/gil/io/detail/filesystem.hpp

Lines 14 to 21 in 9ecdb87

	#if !defined(BOOST_NO_CXX17_HDR_FILESYSTEM) || defined(__cpp_lib_filesystem)
	#include <filesystem>
	#define BOOST_GIL_IO_USE_STD_FILESYSTEM
	#elif defined(__cpp_lib_experimental_filesystem)
	#include <experimental/filesystem>
	#define BOOST_GIL_IO_USE_STD_FILESYSTEM
	#define BOOST_GIL_IO_USE_STD_EXPERIMENTAL_FILESYSTEM
	#endif

and for simple_all_formats test build for C++11 and C++14, this case should happen:

gil/include/boost/gil/io/detail/filesystem.hpp

Lines 36 to 38 in 9ecdb87

	#define BOOST_FILESYSTEM_VERSION 3
	#include <boost/filesystem.hpp>
	#define BOOST_GIL_IO_USE_BOOST_FILESYSTEM

[mloskot]

LGTM. Thanks!

And the CI-s are green now