Install version/config files under CMAKE_INSTALL_LIBDIR

Prior to the c_master/cpp_master split, this is where the files were
installed (c.f., https://github.com/msgpack/msgpack-c/blob/6e7deb809120881634b3ca895e66b2a946084f34/CMakeLists.txt#L454)
but this was changed to use "lib/" directly when updating the C++-only
packaging.

Re-instate the use of CMAKE_INSTALL_LIBDIR so the user can control where
they are installed and they follow the typical convention for location.

Author: jamessan

CMakeLists.txt

@@ -211,7 +211,7 @@ INCLUDE (CMakePackageConfigHelpers)
 IF (NOT (CMAKE_VERSION VERSION_LESS 3.14))
     SET (extra_version_file_args ARCH_INDEPENDENT)
 ENDIF ()
-SET (cmake_config_path "lib/cmake/msgpackc-cxx")
+SET (cmake_config_path "${CMAKE_INSTALL_LIBDIR}/cmake/msgpackc-cxx")
 
 # Configure the main package file from source tree.
 CONFIGURE_PACKAGE_CONFIG_FILE (