add secure_getenv function

This function is a GNU extension introduced in glibc 2.17.

include/stdlib.h

@@ -152,6 +152,7 @@ int ptsname_r(int, char *, size_t);
 char *ecvt(double, int, int *, int *);
 char *fcvt(double, int, int *, int *);
 char *gcvt(double, int, char *);
+char *secure_getenv(const char *);
 struct __locale_struct;
 float strtof_l(const char *__restrict, char **__restrict, struct __locale_struct *);
 double strtod_l(const char *__restrict, char **__restrict, struct __locale_struct *);

src/env/secure_getenv.c

+#define _GNU_SOURCE
+#include <stdlib.h>
+#include "libc.h"
+
+char *secure_getenv(const char *name)
+{
+	return libc.secure ? NULL : getenv(name);
+}