There are six header files installed:
cmark-gfm /usr/include/cmark.h
cmark-gfm /usr/include/cmark_export.h
cmark-gfm /usr/include/cmark_extension_api.h
cmark-gfm /usr/include/cmark_version.h
cmark-gfm /usr/include/cmarkextensions_export.h
cmark-gfm /usr/include/core-extensions.h
But in the file cmark_extension_api.h, two includes of other header files are present:
#include <render.h>
#include <buffer.h>
This makes the file unusable when building as a shared library, along with core-extensions.h which includes it.