AM_GNU_GETTEXT_INTL_SUBDIR macro specifies that the
AM_GNU_GETTEXT macro, although invoked with the first argument
‘external’, should also prepare for building the intl/
AM_GNU_GETTEXT_INTL_SUBDIR invocation can occur before or after
AM_GNU_GETTEXT invocation; the order doesn’t matter.
The use of this macro requires GNU automake 1.10 or newer and GNU autoconf 2.61 or newer.