Add getpgid() placeholder for Minix

c/os_minix.c

@@ -1,3 +1,4 @@
+#include <errno.h>
 #include <math.h>
 #include <setjmp.h>
 #include <stdarg.h>
@@ -8,6 +9,13 @@
 
 #include "scheme.h"
 
+pid_t getpgid(pid_t pid)
+{
+    (void)pid;
+    errno = ENOSYS;
+    return (pid_t)-1;
+}
+
 char *get_exename(char *buf, size_t size)
 {
     (void)buf;

c/os_unix.c

@@ -19,6 +19,8 @@
 
 #include "scheme.h"
 
+pid_t getpgid(pid_t pid);
+
 static value_t dirsym;
 struct fltype *dirtype;