build: Make shared library dependencies work in relocatable installation.
* gettext-tools/gnulib-lib/Makefile.am (RELOCATABLE_LIBRARY_PATH): New variable.
(libgettextlib_la_LDFLAGS): Augment for relocatable installation.
* gettext-tools/src/Makefile.am (libgettextsrc_la_LDFLAGS): Augment for
relocatable installation.
* gettext-tools/libgettextpo/Makefile.am (RELOCATABLE_LIBRARY_PATH): New
variable.
(libgettextpo_la_LDFLAGS): Augment for relocatable installation.
