Use CMAKE_INSTALL_LIBDIR in CONF_CMAKE_INSTALL_DIR (mfontanini#472)

On some platforms (like RedHat ones), CMAKE_INSTALL_LIBDIR is set to
`lib64` instead of `lib`. The CMake files should also be installed to
`lib64`, but because CONF_CMAKE_INSTALL_DIR is set unconditionally to
use `lib`, the proper path can't be configured.

This change makes CONF_CMAKE_INSTALL_DIR use the configured
CMAKE_INSTALL_LIBDIR value, which defaults to `lib`.

CMakeLists.txt

@@ -318,7 +318,7 @@ ENDIF()
 # **********************************
 
 if(UNIX)
-    set(CONF_CMAKE_INSTALL_DIR lib/cmake/libtins)
+    set(CONF_CMAKE_INSTALL_DIR "${CMAKE_INSTALL_LIBDIR}/cmake/libtins")
 else()
     set(CONF_CMAKE_INSTALL_DIR CMake)
 endif()