tests: fix handling of gettext macros requirement

tests: fix handling of gettext macros requirement

The code introduced in the earlier change `v1.11-581-gb7d67d5'
and aimed at automatically fetching all the .m4 files provided by
gettext has proven inadequate, since it hasn't managed to truly
and always get *all* the required .m4 files.  For example, it has
failed to fetch the file `intldir.m4' (present in gettext 0.18.1,
and where the macro `AM_GNU_GETTEXT_INTL_SUBDIR' is defined),
which in turn has caused spurious SKIPs of at least the test
`gettext3.test'.

The reason for this debacle is that autopoint looks at the version
specified in AM_GNU_GETTEXT_VERSION to decide which version of the
gettext infrastructure to bring in; since we were unconditionally
specifying the older 0.10.35 version, the newer `.m4' files weren't
brought in.

* tests/gettext-macros.test: In AM_GNU_GETTEXT_VERSION, instead
of unconditionally specifying the older 0.10.35 version, specify
the version of the available `gettextize' or `autopoint' program,
dynamically extracted with the help of ...
(extract_program_version):... this new function, and saved ...
($autopoint_version): ... in this new variable.