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

fusl/src/env/setenv.c

+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+
+int __putenv(char *s, int a);
+
+int setenv(const char *var, const char *value, int overwrite)
+{
+        char *s;
+        int l1, l2;
+
+        if (!var || !*var || strchr(var, '=')) {
+                errno = EINVAL;
+                return -1;
+        }
+        if (!overwrite && getenv(var)) return 0;
+
+        l1 = strlen(var);
+        l2 = strlen(value);
+        s = malloc(l1+l2+2);
+        if (s) {
+                memcpy(s, var, l1);
+                s[l1] = '=';
+                memcpy(s+l1+1, value, l2);
+                s[l1+l2+1] = 0;
+                if (!__putenv(s, 1)) return 0;
+        }
+        free(s);
+        return -1;
+}