Define a fallback PATH_MAX if not available

PATH_MAX is optional in POSIX, and it is not available on GNU/Hurd.
While it could be possible to not rely on PATH_MAX, for now provide a
fallback definition (which should be safe enough) to get quickjs built
on GNU/Hurd.

quickjs-libc.c

@@ -78,6 +78,10 @@ typedef sig_t sighandler_t;
 #include "list.h"
 #include "quickjs-libc.h"
 
+#if !defined(PATH_MAX)
+#define PATH_MAX 4096
+#endif
+
 /* TODO:
    - add socket calls
 */