build: Remove unnecessary enabling of target property: MACOSX_RPATH

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.

src/CMakeLists.txt

@@ -28,7 +28,6 @@ add_library(cmark
   xml.c)
 cmark_add_compile_options(cmark)
 set_target_properties(cmark PROPERTIES
-  MACOSX_RPATH TRUE
   OUTPUT_NAME "cmark"
   # Avoid name clash between PROGRAM and LIBRARY pdb files.
   PDB_NAME libcmark