Runtime detection of working UTMP/WTMP support in system

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/util.c

@@ -30,6 +30,7 @@
 #include <stdlib.h>
 #include <time.h>
 #include <unistd.h>
+#include <utmp.h>
 #include <sys/sysinfo.h>	/* sysinfo() */
 #include <lite/lite.h>		/* strlcat() */
 #include "util.h"
@@ -75,6 +76,16 @@ int echo(char *file, int append, char *fmt, ...)
 	return 0;
 }
 
+/*
+ * musl libc default to /dev/null/utmp and /dev/null/wtmp, respectively.
+ * See https://www.openwall.com/lists/musl/2012/03/04/4 for reasoning.
+ * Also, there's no __MUSL__, so we cannot make a libc-specific #ifdef
+ */
+int has_utmp(void)
+{
+	return strncmp(_PATH_UTMP, "/dev/null", 9);
+}
+
 int strtobytes(char *arg)
 {
 	int mod = 0, bytes;

src/util.h

@@ -40,6 +40,7 @@ extern char *prognm;
 
 char *progname     (char *arg0);
 int   echo         (char *file, int append, char *fmt, ...);
+int   has_utmp     (void);
 int   strtobytes   (char *arg);
 void  do_sleep     (unsigned int sec);