*configure.ac: Fix warning about 'AM_PROG_MKDIR_P' macro being deprecated

At present,

$ ./bootstrap
...
configure.ac:284: warning: The 'AM_PROG_MKDIR_P' macro is deprecated, and its use is discouraged.
configure.ac:284: You should use the Autoconf-provided 'AC_PROG_MKDIR_P' macro instead,
configure.ac:284: and use '$(MKDIR_P)' instead of '$(mkdir_p)'in your Makefile.am files.
...

The warning comes from the AM_GNU_GETTEXT macro which uses AM_PROG_MKDIR_P. As per this[0] bug report this has been fixed in gettext 0.18.2 so bumped the required gettext version to 0.18.2 from 0.18.1

[0] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=709740