Add #ifdef, comment, and update libc readme

Author: hoodmane

system/lib/libc/README.md

@@ -12,6 +12,7 @@ Some changes have been made to the version that was taken from upstream, includi
  * Switch to using the wasi `fd_write` syscall instead of `writev`.
  * Simplify stdout stream handling: do not support seeking, terminal handling, etc., as it just increases code size and Emscripten doesn't have those features anyhow.
  * Setting `_POSIX_REALTIME_SIGNALS` and `_POSIX_SPAWN` macros to -1, to exclude unsupported functions.
+ * Handling trailing % in `strftime` and `wcsftime` format strings.
 
 Copy log.c and log2.c from earlier version of musl which result in smaller
 binary size since they do not rely data tables in log_data.c and log2_data.c.

system/lib/libc/musl/src/time/strftime.c

@@ -226,7 +226,13 @@ size_t __strftime_l(char *restrict s, size_t n, const char *restrict f, const st
 			s[l] = 0;
 			return l;
 		}
+#ifdef __EMSCRIPTEN__
+		// Handle trailing % by outputting a % rather than returning 0. Ideally
+		// this 6 character change could be upstreamed into musl...
 		if (*f != '%' || !f[1]) {
+#else
+		if (*f != '%') {
+#endif
 			s[l++] = *f;
 			continue;
 		}