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

fusl/src/process/posix_spawnp.c

+#include <spawn.h>
+#include <unistd.h>
+
+int __execvpe(const char *, char *const *, char *const *);
+
+int __posix_spawnx(pid_t *restrict, const char *restrict,
+        int (*)(const char *, char *const *, char *const *),
+        const posix_spawn_file_actions_t *,
+        const posix_spawnattr_t *restrict, char *const *restrict, char *const *restrict);
+
+int posix_spawnp(pid_t *restrict res, const char *restrict file,
+        const posix_spawn_file_actions_t *fa,
+        const posix_spawnattr_t *restrict attr,
+        char *const argv[restrict], char *const envp[restrict])
+{
+        return __posix_spawnx(res, file, __execvpe, fa, attr, argv, envp);
+}