build: Make pam module installation dir configurable

And avoid treating "libdir" as an absolute path, the documentation
clearly states that it is "relative to the prefix".

Based on patch by Timothy Gu <timothygu99@gmail.com>

meson_options.txt

@@ -12,6 +12,9 @@ option('systemd_system_unit_dir',
 option('dbus_service_dir',
     description: 'Directory for dbus service files',
     type: 'string')
+option('pam_modules_dir',
+    description: 'Directory for PAM modules',
+    type: 'string')
 option('gtk_doc',
     type: 'boolean',
     value: false,

pam/meson.build

 mapfile = files('pam_fprintd.ver')
-pam_modules_dir = '/' + get_option('libdir') / 'security'
+pam_modules_dir = get_option('pam_modules_dir')
+if pam_modules_dir == ''
+    pam_modules_dir = '/' / get_option('libdir') / 'security'
+endif
 
 pam_fprintd = shared_module('pam_fprintd',
     name_prefix: '',