build: mkldnn compat: put symlinks together with generate lib

CMake allows overriding the place where the libraries are being
generated using:

``` cmake
set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY some_place)
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY some_place)
```

In this case, the `libdnnl.so` will be put in `some_place` instead of
usual `${CMAKE_CURRENT_BINARY_DIR}` location.

However, we generate the compat symlinks in the latter location. This
makes them point to non-existent library, which in turn:
1. Invalidates the symlink;
2. Betrays users expectations on find the library (which is though
   a symlink) in the proper location `some_place`.
3. Makes `make` regenerate the symlink all the time no matter whether
   it exists or not. That happens because `make` checks whether the
   target (symlink) is _older_ than the prerequisite (`libdnnl.so`).
   It seems that the date for symlink is checked by the date of the
   object it points to. Since the symlink points to nowhere, the
   target is considered always outdated.

This closes #743.

Author: Fomenko, Evarist M

src/CMakeLists.txt

@@ -164,15 +164,19 @@ else()
     endif()
     foreach(ver "" ${vers})
         set_ternary(ext_and_ver APPLE "${ver}${ext}" "${ext}${ver}")
-        add_custom_command(OUTPUT libmkldnn${ext_and_ver}
+        get_property(lib_location TARGET ${LIB_NAME} PROPERTY LIBRARY_OUTPUT_DIRECTORY)
+        if(lib_location)
+            set(compat_link "${lib_location}/libmkldnn${ext_and_ver}")
+        else()
+            set(compat_link "${CMAKE_CURRENT_BINARY_DIR}/libmkldnn${ext_and_ver}")
+        endif()
+        add_custom_command(OUTPUT ${compat_link}
             # to make the next command work fine
-            COMMAND ${CMAKE_COMMAND} -E remove -f libmkldnn${ext_and_ver}
-            COMMAND ${CMAKE_COMMAND} -E create_symlink libdnnl${ext_and_ver} libmkldnn${ext_and_ver}
+            COMMAND ${CMAKE_COMMAND} -E remove -f ${compat_link}
+            COMMAND ${CMAKE_COMMAND} -E create_symlink libdnnl${ext_and_ver} ${compat_link}
             DEPENDS ${LIB_NAME})
-        add_custom_target(compat_libs${ver} ALL
-                DEPENDS libmkldnn${ext_and_ver})
-        install(FILES ${CMAKE_CURRENT_BINARY_DIR}/libmkldnn${ext_and_ver}
-            DESTINATION ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR})
+        add_custom_target(compat_libs${ver} ALL DEPENDS ${compat_link})
+        install(FILES ${compat_link} DESTINATION ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR})
     endforeach()
 endif()