Use CMake's ${CMAKE_INSTALL_LIBDIR} variable to decide where to place the built libraries

The typical situation where the usual, plain 'lib' is not wanted is in a
multiarch system, that allows installation of the same library built for
multiple architectures.
Use the standard ${CMAKE_INSTALL_LIBDIR} variable (coming from the
GNUInstallDirs module) to figure where to put the built libraries.
Signed-off-by: Mattia Rizzolo <mattia@mapreri.org>

CMakeLists.txt

@@ -415,7 +415,8 @@ else()
     ocv_update(OPENCV_OTHER_INSTALL_PATH   etc)
     ocv_update(OPENCV_CONFIG_INSTALL_PATH  ".")
   else()
-    ocv_update(OPENCV_LIB_INSTALL_PATH     lib${LIB_SUFFIX})
+    include(GNUInstallDirs)
+    ocv_update(OPENCV_LIB_INSTALL_PATH     ${CMAKE_INSTALL_LIBDIR}${LIB_SUFFIX})
     ocv_update(OPENCV_3P_LIB_INSTALL_PATH  share/OpenCV/3rdparty/${OPENCV_LIB_INSTALL_PATH})
     ocv_update(OPENCV_SAMPLES_SRC_INSTALL_PATH    share/OpenCV/samples)
     ocv_update(OPENCV_JAR_INSTALL_PATH share/OpenCV/java)
@@ -424,7 +425,7 @@ else()
     if(NOT DEFINED OPENCV_CONFIG_INSTALL_PATH)
       math(EXPR SIZEOF_VOID_P_BITS "8 * ${CMAKE_SIZEOF_VOID_P}")
       if(LIB_SUFFIX AND NOT SIZEOF_VOID_P_BITS EQUAL LIB_SUFFIX)
-        ocv_update(OPENCV_CONFIG_INSTALL_PATH lib${LIB_SUFFIX}/cmake/opencv)
+        ocv_update(OPENCV_CONFIG_INSTALL_PATH ${CMAKE_INSTALL_LIBDIR}${LIB_SUFFIX}/cmake/opencv)
       else()
         ocv_update(OPENCV_CONFIG_INSTALL_PATH share/OpenCV)
       endif()