According to CMake's documentation, MACOSX_RPATH is on already:

  https://cmake.org/cmake/help/latest/policy/CMP0042.html

Specifically:

  CMake 3.0 and later prefer this property to be ON by default.
  [This policy] may be set by cmake_policy() or cmake_minimum_required().
  If it is not set, CMake warns, and uses OLD behavior.

Well, the top-level 'CMakeLists.txt' already sets the minimum version:

  cmake_minimum_required(VERSION 3.7)

Therefore, it is unnecessary to enable it explicitly.

…ibcmark.so'

Before this commit, the 'cmark' executable didn't inform the
dynamic linker where to look for the 'libcmark' shared object;
this becomes an irritation when libcmark is installed in an
unorthodox location:

  $ ldd /path/to/installed/files/bin/cmark
          linux-gate.so.1 (0xb7ed7000)
          libcmark.so.0.31.0 => not found
          libc.so.6 => /usr/lib/libc.so.6 (0xb7cf3000)
          /lib/ld-linux.so.2 => /usr/lib/ld-linux.so.2 (0xb7ed8000)

Because of this commit, the cmark executable's 'rpath' setting
(or equivalent) is set upon installation, thereby providing the
required search directory:

  $ ldd /path/to/installed/files/bin/cmark
          linux-gate.so.1 (0xb7ef2000)
          libcmark.so.0.31.0 => /path/to/installed/files/lib/libcmark.so.0.31.0 (0xb7e95000)
          libc.so.6 => /usr/lib/libc.so.6 (0xb7cbc000)
          /lib/ld-linux.so.2 => /usr/lib/ld-linux.so.2 (0xb7ef3000)

There is some intelligence behind whether rpath is set at all:

  * If 'CMAKE_INSTALL_RPATH' is set, then it is used; this
    allows the user to set a single, overriding value in
    the expected way.

  * Otherwise, the base case rpath is determined by the following:

      "${CMAKE_INSTALL_FULL_LIBDIR}"

    If that path is a standard location, then the base case rpath
    is the empty string; otherwise, the base case rpath is that
    path.

  * The cmark executable's rpath is set to the base case rpath.

  * Currently, libcmark's rpath is set to the empty string.

  * In addition, cmake has been instructed to add to each target's
    rpath any directories that it thinks might contain potential
    dependencies from outside the project; most of the time, there
    will be none, and so nothing will be added.

Of course, this will only help on a system that supports an rpath
feature known to cmake; for example, Windows has no such feature,
and so all of this will presumably be ignored when building under
that system.