md4c is vendored vanilla (0.5.2) under deps_src/md4c and otherwise built shared
(the slicer's global BUILD_SHARED_LIBS overrides its local option) but not
installed -- so the linked libmd4c-html.so resolves against the system md4c at
runtime after the install-time rpath strip. Build against the system
dev-libs/md4c (0.5.3, API-compatible) instead: drop the vendored subdir and
expose INTERFACE shims under the same target names the slicer links, so
target_link_libraries(... md4c-html ...) pulls the system library.

--- a/deps_src/CMakeLists.txt
+++ b/deps_src/CMakeLists.txt
@@ -27,5 +27,12 @@
 add_subdirectory(libnest2d)
 add_subdirectory(mcut)
-add_subdirectory(md4c)
+# ::stuff: build against system dev-libs/md4c instead of the vendored copy.
+# Provide INTERFACE shims under the vendored target names so the slicer's
+# target_link_libraries(... md4c-html ...) resolves to the system library.
+find_package(md4c REQUIRED)
+add_library(md4c INTERFACE)
+target_link_libraries(md4c INTERFACE md4c::md4c)
+add_library(md4c-html INTERFACE)
+target_link_libraries(md4c-html INTERFACE md4c::md4c-html)
 add_subdirectory(mdns)
 add_subdirectory(miniz)