Update configure.ac

Fixed autoreconf warning.

configure.ac:731: warning: The 'AM_PROG_MKDIR_P' macro is deprecated, and its use is discouraged.
configure.ac:731: You should use the Autoconf-provided 'AC_PROG_MKDIR_P' macro instead,
configure.ac:731: and use '$(MKDIR_P)' instead of '$(mkdir_p)'in your Makefile.am files.

configure.ac

@@ -737,7 +737,7 @@ fi
 
 # Checks for library functions.
 AM_GNU_GETTEXT([external])
-AM_GNU_GETTEXT_VERSION([0.18])
+AM_GNU_GETTEXT_VERSION([0.19])
 AC_FUNC_ERROR_AT_LINE
 AC_PROG_GCC_TRADITIONAL