pkg/wuffs: build without libc

This modifies our wuffs dependency so that it no longer requires libc.

This unblocks using wuffs from libghostty-vt on freestanding targets,
which we'll eventually want for some Kitty graphics stuff.

pkg/wuffs/include/string.h

@@ -0,0 +1,22 @@
+// Minimal string.h so that wuffs can be translated and compiled without
+// requiring libc headers (e.g. for wasm32-freestanding targets, or to
+// avoid needing an Apple SDK for translate-c on macOS).
+//
+// Wuffs only calls the mem* family, whose symbols are provided by Zig's
+// compiler-rt on targets without libc and by libc everywhere else. The
+// declarations here match the standard C ABI, so this header is safe to
+// use on every target, including ones that do link libc.
+#ifndef GHOSTTY_WUFFS_STRING_H
+#define GHOSTTY_WUFFS_STRING_H
+
+#include <stddef.h>
+
+void *memcpy(void *dst, const void *src, size_t n);
+void *memmove(void *dst, const void *src, size_t n);
+void *memset(void *b, int c, size_t n);
+int memcmp(const void *s1, const void *s2, size_t n);
+size_t strlen(const char *s);
+int strcmp(const char *s1, const char *s2);
+int strncmp(const char *s1, const char *s2, size_t n);
+
+#endif