Replace _XOPEN_SOURCE with _POSIX_C_SOURCE

And make sure we don't define both in the same source file.

main.c

@@ -1,5 +1,4 @@
-#define _XOPEN_SOURCE 700
-#define _POSIX_C_SOURCE 200112L
+#define _POSIX_C_SOURCE 200809L
 #include <assert.h>
 #include <ctype.h>
 #include <errno.h>

pam.c

@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 500
+#define _POSIX_C_SOURCE 200809L
 #include <pwd.h>
 #include <security/pam_appl.h>
 #include <stdbool.h>

password.c

@@ -1,4 +1,3 @@
-#define _XOPEN_SOURCE 500
 #include <assert.h>
 #include <errno.h>
 #include <pwd.h>

render.c

@@ -1,4 +1,3 @@
-#define _POSIX_C_SOURCE 199506L
 #include <math.h>
 #include <stdlib.h>
 #include <wayland-client.h>

shadow.c

@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE
+#define _XOPEN_SOURCE // for crypt
 #include <pwd.h>
 #include <shadow.h>
 #include <stdbool.h>