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 6bb2ad3..d8d2840 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -291,7 +291,7 @@
     install(
         TARGETS ${target}
         EXPORT MbedTLSTargets
-        DESTINATION ${LIB_INSTALL_DIR}
+        DESTINATION ${CMAKE_INSTALL_LIBDIR}
         PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ)
 endforeach(target)