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)

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

  * 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.

CMakeLists.txt

@@ -69,6 +69,17 @@ set(CMAKE_VISIBILITY_INLINES_HIDDEN 1)
 
 set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
+if(BUILD_SHARED_LIBS)
+  set(p "${CMAKE_INSTALL_FULL_LIBDIR}")
+  list(FIND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES "${p}" i)
+  if("${i}" STREQUAL "-1")
+    set(Base_rpath "${p}")
+  endif()
+endif()
+
+# Append non-standard external dependency directories, if any.
+set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
+
 # Check integrity of node structure when compiled as debug
 add_compile_options($<$<CONFIG:Debug>:-DCMARK_DEBUG_NODES>)
 

src/CMakeLists.txt

@@ -55,7 +55,8 @@ add_executable(cmark_exe
   main.c)
 cmark_add_compile_options(cmark_exe)
 set_target_properties(cmark_exe PROPERTIES
-  OUTPUT_NAME "cmark")
+  OUTPUT_NAME "cmark"
+  INSTALL_RPATH "${Base_rpath}")
 target_link_libraries(cmark_exe PRIVATE
   cmark)