configure.ac: Check if __overflow() is available.

configure.ac

@@ -58,6 +58,8 @@ AC_CHECK_FUNCS([memmove setenv],
 AC_CHECK_FUNCS([execvpe])
 dnl These are not in POSIX.
 AC_CHECK_FUNCS([fwrite_unlocked fputs_unlocked fputc_unlocked])
+dnl Internal functions in libc implementations which must be hooked.
+AC_CHECK_FUNCS([__overflow])
 
 dnl Thanks to gperftools' configure.ac (https://code.google.com/p/gperftools).
 AC_MSG_CHECKING([for __builtin_expect])

src/coloredstderr.c

@@ -275,7 +275,7 @@ HOOK_FILE1(int, putchar_unlocked, stdout,
  * part. As writes to stderr are never buffered, __overflow() is always called
  * and everything works fine. This is only a problem if stdout is dupped to
  * stderr (which shouldn't be the case too often). */
-#ifdef HAVE_STRUCT__IO_FILE__FILENO
+#if defined(HAVE_STRUCT__IO_FILE__FILENO) && defined(HAVE___OVERFLOW)
 /* _IO_FILE is glibc's representation of FILE. */
 HOOK_FILE2(int, __overflow, f, _IO_FILE *, f, int, ch)
 #endif