cmake: Use GnuInstallDirs to customize install directories

Replace custom LIB_INSTALL_DIR with standard CMAKE_INSTALL_LIBDIR variable.
For backward compatibility, set CMAKE_INSTALL_LIBDIR if LIB_INSTALL_DIR is set.

Signed-off-by: Biswapriyo Nath <nathbappai@gmail.com>
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index 1b2980d..798f42f 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -239,7 +239,7 @@
             PUBLIC MBEDTLS_USER_CONFIG_FILE="${MBEDTLS_USER_CONFIG_FILE}")
     endif()
     install(TARGETS ${target}
-            DESTINATION ${LIB_INSTALL_DIR}
+            DESTINATION ${CMAKE_INSTALL_LIBDIR}
             PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ)
 endforeach(target)