Fix implicit function declaration in configure logs: mkdir
https://bugs.gentoo.org/898594

--- a/support/cpp/configure.ac
+++ b/support/cpp/configure.ac
@@ -2317,6 +2317,7 @@
 ################################################################################
 
 # mkdir takes a single argument on some systems.
+AC_CHECK_HEADERS([sys/stat.h])
 gcc_AC_FUNC_MKDIR_TAKES_ONE_ARG
 
 # File extensions