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/CMakeLists.txt b/CMakeLists.txt
index 0fccd3d..11ca1c9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -39,6 +39,8 @@
     project("mbed TLS" C)
 endif()
 
+include(GNUInstallDirs)
+
 # Determine if mbed TLS is being built as a subproject using add_subdirectory()
 if(NOT DEFINED MBEDTLS_AS_SUBPROJECT)
   set(MBEDTLS_AS_SUBPROJECT ON)
@@ -249,8 +251,7 @@
 endif(CMAKE_BUILD_TYPE STREQUAL "Coverage")
 
 if(LIB_INSTALL_DIR)
-else()
-    set(LIB_INSTALL_DIR lib)
+    set(CMAKE_INSTALL_LIBDIR "${LIB_INSTALL_DIR}")
 endif()
 
 add_subdirectory(include)