OpenCSG renders constructive solid geometry on the GPU with image-based CSG
algorithms.  openscad uses it for the interactive preview, so it is needed
to build openscad.

Version 1.8.2 was released on 2025-01-05.  Only the library and its single
header are installed; upstream’s example is a GLUT demo that is never
installed, so turn it off and avoid the freeglut dependency:

    /usr/lib/libopencsg.so.1.8.2
    /usr/lib/libopencsg.so.1
    /usr/lib/libopencsg.so
    /usr/include/opencsg.h

Assisted-by: Claude Opus 5 <noreply@anthropic.com>

OpenSCAD is a script-only 3D CAD modeller: the model is described in its
own declarative language and rendered from constructive solid geometry,
making it a good fit for parametric parts and for keeping designs in
version control.

2021.01, released on 2021-02-07, is still the latest release; upstream has
published development snapshots only since then.  The tarball bundles the
MCAD library, so no submodule handling is needed.

MarIuX’ boost is configured with `--with-libraries=headers`, so `/usr/lib`
holds no libboost_* at all and the link fails:

    /usr/lib/gcc/x86_64-pc-linux-gnu/12.5.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lboost_thread: No such file or directory
    /usr/lib/gcc/x86_64-pc-linux-gnu/12.5.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lboost_program_options: No such file or directory
    /usr/lib/gcc/x86_64-pc-linux-gnu/12.5.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lboost_filesystem: No such file or directory
    /usr/lib/gcc/x86_64-pc-linux-gnu/12.5.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lboost_system: No such file or directory
    /usr/lib/gcc/x86_64-pc-linux-gnu/12.5.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lboost_regex: No such file or directory
    collect2: error: ld returned 1 exit status

Take boost from `/pkg/boost-1.86.0-0`, which ships the compiled
libraries.  Its headers have to precede /usr/include so that the 1.86.0
headers, and not the 1.72.0 ones, match the libraries; QMAKE_RPATHDIR
makes them findable at runtime without a profile:

    RPATH                /pkg/boost-1.86.0-0/lib:/usr/local/qt5/lib

Four patches are required, three for the CGAL 5.6.3 and one for the boost
1.86.0 built against:

    src/cgalutils-tess.cc:22:15: error: ‘Triangulation_2_filtered_projection_traits_3’ in namespace ‘CGAL’ does not name a template type; did you mean ‘Triangulation_2_projection_traits_3’?
    src/cgalutils-polyhedron.cc:346:29: error: ‘generic_print_polyhedron’ was not declared in this scope
    src/openscad.cc:310:71: error: call of overloaded ‘join(std::__cxx11::list<std::__cxx11::basic_string<char> >, const char [2])’ is ambiguous
    src/FileModule.cc:68:25: error: ‘is_regular’ is not a member of ‘fs’; did you mean ‘is_regular_file’?

The build has to happen in the source directory.  qmake looks for feature
files only in a `features` directory below the *build* directory, so in a
shadow build openscad’s own features/*.prf – which is where every
dependency except Qt is detected – are silently skipped and the build dies
with

    src/CGALCache.cc:27:10: fatal error: Eigen/Core: No such file or directory
       27 | #include <Eigen/Core>
          |          ^~~~~~~~~~~~

Verified by rendering a CSG difference headlessly, which exercises the exact
CGAL kernel and the STL exporter:

    $ openscad -o t.stl t.scad
    Geometries in cache: 3
    […]
       Facets:        466
       Volumes:         2
    $ grep -c 'facet normal' t.stl
    880

Assisted-by: Claude Opus 5 <noreply@anthropic.com>