gettext-tools: Use a short name for DLLs on OS/2

gettext-tools: Use a short name for DLLs on OS/2

OS/2 does not support a DLL name longer than 8 characters.

* gettext-tools/configure.ac (OS2): Define on OS/2.
* gettext-tools/gnulib-lib/Makefile.am (libgettextlib_la_LDFLAGS):
Set a DLL name to gtlib on OS/2.
* gettext-tools/libgettextpo/Makefile.am (libgettextpo_la_LDFLAGS):
Set a DLL name to gtpo on OS/2.
* gettext-tools/src/Makefile.am (libgettextsrc_la_LDFLAGS): Set a DLL
name to gtsrc on OS/2.