Add a "fork" of musl as //fusl.

fusl/src/env/unsetenv.c

+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+
+extern char **__environ;
+extern char **__env_map;
+
+int unsetenv(const char *name)
+{
+        int i, j;
+        size_t l = strlen(name);
+
+        if (!*name || strchr(name, '=')) {
+                errno = EINVAL;
+                return -1;
+        }
+again:
+        for (i=0; __environ[i] && (memcmp(name, __environ[i], l) || __environ[i][l] != '='); i++);
+        if (__environ[i]) {
+                if (__env_map) {
+                        for (j=0; __env_map[j] && __env_map[j] != __environ[i]; j++);
+                        free (__env_map[j]);
+                        for (; __env_map[j]; j++)
+                                __env_map[j] = __env_map[j+1];
+                }
+                for (; __environ[i]; i++)
+                        __environ[i] = __environ[i+1];
+                goto again;
+        }
+        return 0;
+}