See https://bugs.gentoo.org/935126

--- a/m4/printf.m4
+++ b/m4/printf.m4
@@ -894,6 +894,7 @@ AC_DEFUN([gl_PRINTF_DIRECTIVE_N],
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 #ifdef _MSC_VER
 #include <inttypes.h>
 /* See page about "Parameter Validation" on msdn.microsoft.com.
@@ -1708,6 +1709,7 @@ AC_DEFUN([gl_SNPRINTF_DIRECTIVE_N],
 #include <signal.h>
 #include <stdio.h>
 #include <string.h>
+#include <unistd.h>
 #if HAVE_SNPRINTF
 # define my_snprintf snprintf
 #else