Fix #ifndef statements: HAVE_ variables are all upper case

The autoconf manual states:

   define HAVE_function (in all capitals) if it is available

https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Generic-Functions.html#Generic-Functions

Thanks to @sur5r for the report

libi3/mkdirp.c

@@ -12,7 +12,7 @@
 #include <string.h>
 #include <sys/stat.h>
 
-#ifndef HAVE_mkdirp
+#ifndef HAVE_MKDIRP
 /*
  * Emulates mkdir -p (creates any missing folders)
  *