meson: Define DEFAULT_LIBRARY_PATH as join_paths(get_option('prefix'), get_option('libdir'))

'libdir' defaults to 'lib', so running X -showDefaultLibPath just prints 'lib'
instead of '/usr/lib' or '/usr/local/lib'. Use joint_paths() to get the correct
full path.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>