configure.ac : Add AC_PROG_MKDIR_P.

Closes: https://sourceforge.net/p/flac/bugs/404/
diff --git a/configure.ac b/configure.ac
index d478e10..ffca597 100644
--- a/configure.ac
+++ b/configure.ac
@@ -38,6 +38,7 @@
 XIPH_C_COMPILER_IS_CLANG
 XIPH_GCC_REALLY_IS_GCC
 AC_PROG_MAKE_SET
+AC_PROG_MKDIR_P
 
 AC_SYS_LARGEFILE
 AC_FUNC_FSEEKO