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/3rdparty/everest/CMakeLists.txt b/3rdparty/everest/CMakeLists.txt
index ff9da7a..4ad367e 100644
--- a/3rdparty/everest/CMakeLists.txt
+++ b/3rdparty/everest/CMakeLists.txt
@@ -23,5 +23,5 @@
 
 install(TARGETS everest
   EXPORT MbedTLSTargets
-  DESTINATION ${LIB_INSTALL_DIR}
+  DESTINATION ${CMAKE_INSTALL_LIBDIR}
   PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ)