Move some gettext bits to dxcommon.

Migrate the mostly-package-agnostic parts of the gettext support into
dxcommon, thus allowing these bits to be shared with other packages.

We remove the AC_CONFIG_MACRO_DIR from configure.ac because its
behaviour is not clearly specified, and it's not obvious how (if it's
even possible at all) one handles multiple directories containing macro
definitions.  Moreover, even if this macro is specified you still need
to define ACLOCAL_AMFLAGS anyway, and this definition must be kept
consistent with the AC_CONFIG_MACRO_DIR usage.

In the end, it looks like the only thing AC_CONFIG_MACRO_DIR does is
silence a single warning from libtoolize, without doing anything else
meaningful.  In light of the above, the warning seems preferable at
this time.