build: Tell cmake to set 'rpath' so the installed 'cmark' can find 'l…

…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)

In particular, 'rpath' is set to the following value:

  "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}"

Of course, this will only help on a system that supports an
'rpath' feature known to CMake. Windows has no such feature
anyway, apparently.

CMakeLists.txt

@@ -69,6 +69,8 @@ set(CMAKE_VISIBILITY_INLINES_HIDDEN 1)
 
 set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
+set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}")
+
 # Check integrity of node structure when compiled as debug
 add_compile_options($<$<CONFIG:Debug>:-DCMARK_DEBUG_NODES>)