Add support for building swaylock without PAM

This involves setuid'ing swaylock, which then forks and drops perms on
the parent process. The child process remains root and listens on a pipe
for requests to validate passwords against /etc/shadow.

swaylock/meson.build

@@ -1,25 +1,37 @@
 sysconfdir = get_option('sysconfdir')
 
-executable(
-	'swaylock', [
-		'main.c',
-		'password.c',
-		'render.c',
-		'seat.c'
-	],
+dependencies = [
+    cairo,
+    client_protos,
+    gdk_pixbuf,
+    math,
+    pango,
+    pangocairo,
+    xkbcommon,
+    wayland_client,
+    wlroots,
+]
+
+sources = [
+    'main.c',
+    'password.c',
+    'render.c',
+    'seat.c'
+]
+
+if libpam.found()
+    sources += ['pam.c']
+    dependencies += [libpam]
+else
+    warning('The swaylock binary must be setuid when compiled without libpam')
+    warning('You must do this manually post-install: chmod a+s /path/to/swaylock')
+    sources += ['shadow.c']
+endif
+
+executable('swaylock',
+    sources,
 	include_directories: [sway_inc],
-	dependencies: [
-		cairo,
-		client_protos,
-		gdk_pixbuf,
-		libpam,
-		math,
-		pango,
-		pangocairo,
-		xkbcommon,
-		wayland_client,
-		wlroots,
-	],
+	dependencies: dependencies,
 	link_with: [lib_sway_common, lib_sway_client],
 	install: true
 )